Arena

The Arena is a game mode in which players draft decks to do battle against other players in a tournament-style format for the chance to earn substantial rewards. Players choose cards out of 30 separate selections of cards, building a 30-card deck to do battle against other players. Players play until they have suffered 3 losses or claimed 12 victories, at which point they will be granted a number of rewards based on the final number of wins they achieved.

Unlike in other play modes, cards available for selection are not limited by the player's current collection, and there is no limit on the number of each card that can be included in a deck (including legendaries). Players do not have to play all of their Arena games in one go, and can return to continue their run whenever they wish. Winning at least 7 games before being eliminated guarantees that the player will earn their entry fee back in gold.

To do well in the Arena, players should have a wide knowledge of cards in general and a basic knowledge of card abilities and card interactions. The player should also know how to appropriately draft the strongest cards for their deck based on the cards offered in the draft and on the player's chosen starting class. With a good knowledge of cards, card mechanics, and drafting strategy, a player will be able to do competently well in Arena.

Each admission to the Arena costs either 150 in-game gold or real money equivalent to $1.99 USD, €1.99 EUR, £1.99 GBP, R$4.50 BRL, or ￥12.00 CNY.

With its random selection of heroes, the Arena is locked for players until they have reached level 10 in every class.

Getting started


After paying the Arena entry fee, the player will be presented with three random hero classes to choose from. The player selects a hero to play as in the Arena run, and then the drafting process begins.

In the Arena drafting process, the player is presented with 30 selection screens. For every screen, the player chooses one of three cards to add to their deck. Once a card has been chosen, the player cannot undo their selection. After the player has chosen 30 cards for their deck, the player is then ready to do battle in the Arena.

Unlike in constructed play, where each deck can only feature a maximum of 2 of each card (and 1 of each legendary), there is no limit to the number of each card that can be put in Arena decks. Some cards, especially those of lower rarity, may appear several times in different selection sets. It should also be noted that only neutral cards and cards for the selected class will be offered.

As the player picks cards during the draft, the current decklist is shown on the right of the screen, and the deck's mana curve is displayed at the bottom of the screen, serving to give a quick indication of how the deck is balanced. Once the deck is complete, clicking the "Play" button will queue the player up for a battle against another Arena player.

Because Arena decks feature randomly selected cards and have no involvement with real money expenditure to provide players with advantages, this places all Arena players on a relatively even playing field from the start.

Card selection
There are a number of rules regarding what cards are offered during the Arena drafting process.

The following attempts to list all known rules affecting card selection:
 * The 1st, 10th, 20th, and 30th picks will be guaranteed to offer Rare, Epic, or Legendary cards.
 * This means Legendary cards may or may not be offered for the run.
 * Multi-class cards may be included if they match the player's chosen class.
 * Card selection is not affected by previous card selections, runs, or synergies.

The following are some of the Arena rules from the old Arena Rules forum post in August 2019:
 * Neutral cards from the Basic and Classic sets appear 50% less often.
 * Class cards receive an additional +50% increase in their appearance rate.
 * Spell and weapon cards OR cards from the latest expansion receive an additional +50% increase in their appearance rate.
 * Appearance rate modifiers are multiplicative with one another.

In September 2019, cards shown in Arena selection screens were on average composed of around 79% Common, 15% Rare, 5% Epic, and 1% Legendary cards.

Arena Rotation
On April 9th 2019, with the start of the Year of the Dragon, Arena rotated its draft pool every 2 months, where players drafted cards from select Standard and Wild card sets to keep things fresh. Previously, Arena was played using a Standard-only draft pool.

All active Arena runs ended when the rotation changed. If a player had an active Arena run, they were given the appropriate reward for the number of wins they had, and also a ticket to allow them to start a new Arena run free of charge.

After the introduction of two-month Arena rotations, the micro-adjustment process noted in the section above now occurs typically within a month after a new Arena rotation has taken place.

On April 7th, 2020, Arena rotation returned to Standard for the majority of the Year of the Phoenix. However, it was reinstated on November 17th with the launch of Madness at the Darkmoon Faire.

a Galakrond's Awakening was released and added into Arena on January 21, 2020; however no rotation was implemented.

Card exclusions
The following table lists cards which are excluded from Arena and are not offered in draft picks during the Arena selection process. However, random effects are not affected by exclusions and can still give most of these cards, with the exception of Quest cards which cannot be randomly obtained by any means.

Notable exclusions include Hero cards, Quests, C'Thun synergy cards, Odd/Even synergy cards, and Invoke effects.

The following cards have a reduced likelihood of appearing in draft picks.

Golden cards
Cards presented for selection may be golden or regular depending on the player's collection: Cards in your Arena deck will not be changed if you gain or disenchant a golden card after drafting. You can choose to turn off golden cards for an arena run by clicking a golden card in the decklist, and it cannot be reopened for that run.
 * If the player owns the maximum usable number of golden copies of a card (1 for legendaries, 2 for all other rarities), all copies of that card appearing in drafts will be golden.
 * If the player owns 1 golden copy of a non-legendary card, the first copy of that card appearing in drafts will be golden; the others will be regular.
 * If the player owns no golden copies of a card, all copies of that card appearing in drafts will be regular.

Playing Arena
Play in Arena consists of a series of matches between Arena players, with each player using their specially constructed deck.

During each Arena run the player can only suffer up to three losses, with the number of losses so far displayed on the Arena screen. Once a player has lost 3 times, or won 12 times, their run will end, and rewards befitting their success will be offered. Once the run has ended, the player's deck will be destroyed, and cannot be viewed or played with again.

Players can also choose to retire their deck at any time and claim their reward. However, it should be noted that completing all of a deck's Arena matches cannot reduce the reward on offer, only improve it. Retiring a deck should therefore be a last resort used only when unwilling to play with the deck any longer.

It should also be noted that if either player is disconnected from the server during a match and fails to return through the reconnection feature, that match is counted as a loss for the disconnector.

Matchmaking
Matchmaking for each pairing is determined by the player's win/loss record for the current run, attempting to find a player with as similar a record as possible. Other factors such as class, deck and past Arena performance do not affect matchmaking.

It is possible for a player to be matched twice against the same player over the course of a single Arena run. For example, if player A defeats player B, but then loses their next game, while player B is defeated by player A but then wins their next game, the two players will be a viable match-up for their third games. However, given the large player pools this is very rarely seen.

New players
For players on their first few Arena runs, matchmaking is intentionally altered to provide an easier transition into the game mode. For matchmaking purposes, the player's win/loss ratio is considered to have one more loss than it actually does, thus matching them against what should on average be easier opponents. The system will also prefer to match such players against each other.

The exact number of games required to be eligible for this adjustment is something that the developers are "still tweaking a little", but as of April 2016 is "in the realm of 2 or 3 runs." Once the player has exceeded this number of runs, they are matched purely on their win/loss ratio, with their number of previous runs no longer affecting their matchmaking.

Claiming rewards
Once the player has won 12 games in total, lost 3 games, retired their deck, or a new Arena rotation has taken over, they will be given a Key to open up their rewards chest. The player can be awarded up to five prize boxes, each able to contain gold, dust, cards, and card packs. The range of possible items and values able to be given in each prize box is shown in the rewards table below. Higher-level Keys grant a larger number of prize boxes with more valuable content.

Certain prize boxes are guaranteed for each Key, with the remaining prize boxes determined randomly from one or two reward pools.
 * Example: Reaching 3 wins and getting the Copper key means the player is rewarded 3 prize boxes:
 * (1) One card pack
 * (2) 25-35 gold
 * (3) One random reward from its reward pool


 * Example: Reaching 8 wins and getting the Champion key means the player is rewarded 4 prize boxes:
 * (1) One card pack
 * (2) 150-160 gold
 * (3) One random reward from its first reward pool
 * (4) One random reward from its second reward pool

Rewards table
The below table aims to chart the range of possible rewards for each Key rewarded. The 'No.' column lists the number of prize boxes awarded for each Key.

More valuable items in the random reward pools appear in prize boxes with less probability than other items in the reward pool. For example, the random prize boxes for 12 wins have extremely low chances of giving out legendaries, golden epics, and golden legendaries.


 * Notes
 * Players will always be awarded one card pack regardless of the number of games they have won, and even losing 0-3 will also grant a small amount of gold/dust.
 * Since card packs can be purchased directly from the shop for 100 gold, this essentially makes the additional cost of an Arena run equal to 50 gold, minus the value of gold and dust rewarded from the run – assuming the player desires cards from the latest expansion set. If the player can earn more than this amount back in additional rewards, the run can be considered to have been profitable. Gaining experience in playing Arena can of course also be considered a worthwhile reward, especially in the beginning.
 * Winning at least 7 games will earn the player their entry fee back in gold, in addition to other rewards. This is colloquially known as "going infinite" as the prize gold can be used directly to start another arena run, which could pay for another run at 7 wins, repeating infinitely. Winning 4 to 6 games will compensate for the difference between buying a pack directly and earning one through Arena.


 * Rewards
 * The guaranteed card pack given each run will always be from the most recently released expansion. If the player earns a second card pack from the same run, it will always be from a different Standard format set.
 * Individual card rewards may be common, rare, epic, legendary, and they may be golden or non-golden. Also, they are not limited by class. Individual card rewards may be from either the Classic set, or any expansion set currently in Standard format. Higher rarities/golden versions of individual cards are rewarded at a higher rate at higher level Keys.
 * As of Patch 9.0.0.20457, non-golden common cards are no longer offered as a reward for runs with with three or more wins. Previously, all Keys except the three highest Keys could reward non-golden common cards.
 * Rewards are based solely upon the number of wins for the current run, and are not affected by performance in previous runs.
 * The presence of different rewards can affect the size/quality of the other rewards selected for that player. For example, being awarded a common card will increase the value of the other rewards granted by that Key. This may be due to the likelihood of the player already owning or simply having no desire for any given common card, making it likely they will simply disenchant it, for a mere 5 Arcane Dust.
 * All gold and dust rewards have a range of possible values.
 * The 12-win Key does not reward dust as a prize.

Events
Occasionally, Blizzard will change Arena for a limited time, either by adding exclusive cards or modifying the drafting system.

Players are able to participate in Arena events for free via Arena tickets, which can be obtained by starting up but not finishing their Arena run prior to the date when a new Arena event starts. When a new Arena event is released, any unfinished Arena run that the player has had will end at its final state, and prizes are rewarded to the player based on the player's progress in the unfinished Arena run, and a free Arena ticket is given to the player. Arena tickets allow the player to enter the Arena for free during the Arena event or in any future normal Arena run. Arena tickets will be consumed upon use, but will never expire.

Dual Class Arena
During the Hallow's End (2017 and 2018) and Doom in the Tomb (2019) events, players are able to play as two classes at the same time. The first pick adds the class's cards to the hero pool while the second pick also chooses the Hero Power. Heroes in arena also have modified art, depicting them wearing costumes.

Taverns of Time
For (Jun 11th-July 2nd, 2018), 28 new Arena-exclusive cards are able to be drafted, 2 for each class and 10 neutral cards.

Wildfest
During Wildfest, Wild cards were able to be drafted.

Arena-exclusive cards
From March 11, 2018 through April 10th 2018, each class gained one Arena-exclusive card that was able to be drafted. The cards were chosen by the community from a vote between three class cards.

Goblins vs. Gnomes sneak peek
Before the release of Goblins vs. Gnomes expansion, cards from the set were able to be drafted in Arena.

Strategy
The strategy in Arena is considerably different from constructed play. The emphasis in "draft" mode is mainly on knowing which cards are best to select based on prior cards selected during the draft, and on the pool of cards available in the draft. Players should have a good grounding in the game and the capabilities of each class before entering the Arena, as the admission charge can make Arena play costly for inexperienced players.

There are three key differences between Arena and constructed play.

Firstly, players must select their cards in a fairly random process, and this unpredictability of deck building makes constructing strong synergy decks less likely to occur. This means that players shouldn't worry too much on the opponent playing the same strong synergy plays that are often seen in constructed for their class. The player should worry more about gaining a strong foothold on the board every turn and maintaining board control.

Secondly, players cannot necessarily deduce from their opponent's cards the style of the rest of their deck; however, if the opponent plays many small swarming minions early on, it can indicate that their deck is aggro; and if the opponent skips their first few turns by only hero powering, it may indicate that their deck is more control.

Thirdly, the player's deck is not limited to 2 of each card. This might not be the most impactful, but it allows players the small possibility of building outlandish decks, such as a mage with 2x Archmage Antonidas, a warlock with 4x Twisting Nether, or a priest with 5x Temple Enforcer. This is very unlikely if the number of card sets available in Arena is high, but it is not totally impossible and is somewhat worth keeping in mind.

Rankings
The top 200 Arena players in the Americas, Europe, and Asia regions every month are sometimes published in blog posts on the official site for the corresponding region. Players are ranked by their average wins per run, with a minimum of 30 runs. Arena end-of-month leaderboards use a player's best 30 consecutive runs as their average.

Statistics
Arena competition presents a very specific pattern of player elimination. As each player must either win or lose each game to proceed to the next match, it is possible to calculate the specific percentiles and distribution across the possible win/loss records. This allows players to mathematically rank their run in the Arena according to the percentage of players who have achieved the same record.

Note that these statistics do not represent the player's actual chances of reaching any number of wins. Success in Arena is substantially determined by deck construction and skill in playing each match. The numbers below are predictions of the proportion of players who reach each number of wins. While these numbers reflect the intended design, for reasons of shortening queue times it is possible players will not always be matched against players with precisely the same score.

Number of wins
Players complete their Arena runs with between 0 and 12 wins, with the number of wins directly determining the scale of the prizes awarded.

The below table lists the percentage of runs that reach each number of wins. For example, Arena runs achieving 5 wins are in the top 22.66% of runs. 50% of Arena runs will achieve 3 wins or more.

Exact sequence of matches
Each Arena run features a potential of 14 matches for each player, with runs ending upon reaching 3 losses or 12 wins. In the ideal scenario, players either win or lose each match, dividing players between one of two possible paths for each match.

The below table shows the percentage of runs that will reach each win-loss state, with numbers rounded off to two decimal places. The numbers in bold indicate possible final scores, assuming the player does not retire their deck. For example, we can see that only 0.02% of runs end at 12-0, and that the most common score is either 1-3 or 2-3, with more than a third of all Arena runs ending with one of these scores.

2016 statistics
In October 2016, official statistics were released for Arena play between January and September 2016. A series of infographics listed the top players in each region, followed by some global statistics. The original infographics can be found here.

The performance of individual players was broken down by region, and by a number of criteria. Some of these statistics are collated for all regions below. As a rule China and Asia tallied higher numbers of total wins, runs and 12-win runs, compared to the Europe and Americas regions. The individual class records were mostly claimed by the Americas region, while the Europe region dominated in areas related to average run performances.
 * Top players


 * Highest average number of wins per run: Caesar (EU) with 8.090
 * The second and third highest also went to the EU with Pollitos at 7.991 and Woett at 7.692
 * Highest number of 12-win runs in total: ggcnm (China) with 152 12-win runs
 * Highest number of wins in total: ggcnm (China) with 8,378 wins
 * Longest streak of 7+ win runs: Woett (EU) with 17


 * Notes
 * The players with the highest average number of wins per run were selected from those with at least 100 runs. Designer Dean Ayala states that there were players with higher averages, but all below 100 runs, and mostly below 25 runs.
 * Although not mentioned above, the Americas player with the second highest average number of wins per run was BattlePants, which is actually a 'smurf' account belonging to well-known streamer Hafu.

The following stats were presented collated for all regions, for January-September 2016 period. Additional stats can be found in the original infographics.
 * Overall stats


 * Number of hours spent in Arena games: 157,872,547 hours
 * Number of turns: 6.31 billion turns
 * Most drafted cards:
 * Common: Piloted Shredder, Jeweled Scarab, Tomb Spider
 * Rare: Sludge Belcher, Azure Drake, Sunwalker
 * Epic: Piloted Sky Golem, Kodorider, Grand Crusader
 * Legendary: Dr. Boom, Arch-Thief Rafaam, Ragnaros the Firelord
 * Least drafted:
 * Common: Warsong Commander, Moonfire, Totemic Might
 * Rare: Savagery, Cursed Blade, Darkshire Librarian
 * Epic: Siltfin Spiritwalker, Embrace the Shadow, Tentacles for Arms
 * Legendary: Acidmaw, Nat, the Darkfisher, Herald Volazj
 * Most of one card in a single deck: 9 copies. A Light in the Darkness is shown, but there may be other examples.


 * Notes
 * The Arena run with 9 copies of A Light in the Darkness went 3-3.

Design
The design of the Arena provides for a very different way of playing Hearthstone than that found in constructed play. In comparison, Arena rewards players based more on their ability to construct decks from a less than ideal selection of cards, and to improvise in unlikely match circumstances. It also features a greater emphasis on basic gameplay skills, rather than complex strategies and specific gimmicks. Arena is far less dependent upon knowledge of the current meta, and sees far fewer highly organised decks, but frequently features unlikely, improvised and sometimes extraordinary plays which can be extremely hard to predict.

Arena also provides a second chance for cards rarely seen in constructed play. Many cards widely considered poor choices for constructed decks are presented to players as Arena picks, and end up finding their way into players' decks. Because of this, the diversity of cards seen in Arena is far greater than that of constructed, again contributing to a greater focus on improvisation in response to unexpected circumstances. This is one answer to the often asked questions regarding the existence of certain seemingly poor cards; while they may rarely be chosen in constructed, their presence in Arena broadens the variety of the game mode, and allows it to offer almost an additional set of cards to that seen in constructed play. Not only does their presence provide a greater range of options, but due to the random nature of Arena picks, these otherwise panned cards have the opportunity in Arena to become valuable and even game-winning choices, due in part to the other unlikely cards presented to players when constructing their decks.

Because of the far greater difficulty in constructing a deck with a specific design, knowledge of the current meta, and the ability to play around a central gimmick or specific strategy are far less valuable in the Arena. Improvisational skills are highly important, both in responding to an unpredictable opponent and in playing a less than ideally constructed deck. Players cannot rely on a common sense expectation of what the opponent's deck should hold, nor on a consistent or balanced deck of their own. While "net-decking" the latest top-ranked decklists can provide great advantage in constructed, Arena is far more rewarding of a knowledge of the constituent parts of a deck, and the many ways in which they can be combined when ideal opportunities fail to present themselves.

For these reasons, a different type of player can expect to shine in Arena than in constructed play. While it is possible for players to excel at both, many will find themselves consistently seeing more success in one type of play than in the other. Players with a greater knowledge of the current meta and a focus upon refining specific decks and strategies will likely fare better in constructed, while those less well-versed in the latest trends may have a better chance in the Arena. The lack of opportunity for ideal deck construction allows players with less advanced deck building knowledge a greater chance of success, with familiarity with the latest decklist less valuable than a shrewd instinct for the basic building blocks of the game.

Arena can also provide a break from the relatively construction-focused domain of constructed play. While success in constructed often requires constant tweaks to a deck, and may punish players who do not keep an eye on the latest developments in the meta, a deck made in Arena cannot be changed, and once built must be played as is for better or worse. This can allow players to simply enjoy doing their best under less than ideal circumstances, without excessive focus on where they could have improved the deck, especially considering the great variety between the cards offered in each Arena run.

Arena also gives players a chance to experience many interesting and hard to obtain cards which they may not have the opportunity to play with in the rest of the game. While higher rarity cards are relatively uncommon picks, Arena is the only mode besides the limited realm of adventures' Class Challenges (and certain Tavern Brawls) in which players can play with cards that they do not actually own. This can provide great insight into which cards to craft, or simply highlight the fun of playing with cards the player has never chosen to experiment with. The variety of classes found in Arena can also give players an opportunity to experience playing with classes they do not often choose, as well as sampling those classes' higher rarity cards and higher basic cards, which the player may not yet have earned.

Development


The Arena - originally titled 'The Forge' - was first conceived as a way to incorporate 'draft mode' style play into the game. Drafting with a physical CCG involved players passing round packs of cards, drawing individual cards until they had each built a deck - something many of the developers enjoyed, but which would be difficult to implement within Hearthstone. To solve this problem, the developers implemented asynchronous drafting, allowing each player to separately - yet randomly - build, or 'forge' a unique deck.
 * Initial

Early versions of the Forge had players keeping all the cards they drew for their deck. Admission cost several card packs, and would win packs in exchange for achieving wins. One snapshot of the development process shows the player earning a pack for each win above 4, with 10, 15 and 20 wins granting 5, 15, and 30 additional packs.

The developers eventually decided to remove both the card pack admission cost, and the reward of keeping the chosen cards. One reason for this was to remove the conflict between whether to choose the card that was better for the current Arena run, or the card which the player ultimately wanted to add to their collection. This change allowed players to focus purely on building the best possible Arena deck. Another reason for the change was to remove restrictions on which cards were presented. When the player kept the chosen cards, it was necessary for the range of cards presented for selection to match those which would have been obtained if the player had simply opened the card packs spent to enter the Arena. This ensured the result was fair, but also tied the developers' hands for Arena balance. By no longer allowing players to keep the cards chosen, the developers were free to adjust the balance of rarity to make more exciting Arena runs, as well as including cards from other sets, where before only Classic (then called Expert) cards were featured.

The change also removed the necessity for players to choose whether to open their card packs or spend them to gain access to the Arena, where previously spending card packs might see players unable to keep cards they would have kept if they had simply opened the packs. While players still had to choose where to spend their gold (and real money), the change removed the "horrible tension" of this choice, allowing players to simply enjoy opening card packs without these concerns.

These changes were accompanied by the change of name from the Forge to the Arena. The change was in response to feedback from internal testing, where players often mistook the Forge for the game's crafting mode, believing it was where you went to "forge" new cards. While the term accurately described "forging" a new deck, it was also confusing in that it didn't evoke combat or battle against other players, which was ultimately the main focus of the mode. The new name "The Arena" was chosen to clearly indicate the gladiatorial nature of the mode.

The rewards for the Arena were also iterated upon. Prior to the shift from the Forge to the Arena, rewards came in the form of card packs, directly compensating players for the admission fee. A screenshot from early 2013 shows players earning one card pack for every win, starting with their fifth victory. Extra card packs were awarded upon reaching certain milestones, with reaching 20 wins awarding a bonus of 30 additional card packs. The milestones stated also reflect the Forge's lack of limit to the length of runs in the game mode's early versions. The arrival of the Arena saw the key system introduced, with each win earning a new key, and a maximum of 9 wins. This was later increased to a maximum of 12 in December 2013.

Following the Arena's emergence in the late alpha, and during most of the beta, the fixed system of card pack rewards was exchanged for a number of smaller random rewards. In early Arena builds the number of rewards did not vary dependent on the number of wins, only the contents of the rewards themselves. A player with only 1 win would still win 5 rewards, but these might consist only of 5 gold, 5 gold, 5 gold, 10 dust and a card pack. This was changed in December 2013's Patch 1.0.0.4217, with both the number and quality of rewards now scaling with wins. The exact rewards have since been tweaked a number of times, but this general scheme has remained in place.

All new cards are designed to take into account both Play mode and Arena games. However, the developers acknowledge that some cards "get way better" in Arena than in Play mode, and vice versa.
 * Ongoing

Card exclusions
The first cards to be excluded from the Arena were those of the Promo set, namely Gelbin Mekkatorque and Elite Tauren Chieftain, likely due to their initially being available only through special promotions. With only two cards excluded, and both of those rarely seeing play in any game mode, Arena essentially used the full card pool for the first 2-3 years of the game's life.

It was not until the release of Whispers of the Old Gods in April 2016 that the first major exclusions were made, with C'Thun and all related cards being specifically excluded from the drafting process. These were excluded due to their specific synergy, with the likelihood of drafting enough such cards to prove effective being extremely low.

On August 8, 2016, after receiving negative feedback over the upcoming Purify and the weakness of the priest class in the Arena, Ben Brode announced that Purify would not appear in Arena drafts. This marked the first time a card had been excluded due to Arena balance concerns over its poorness, as opposed to the general lack of synergy of the C'Thun cards.

While the exclusion helped to mitigate frustration over Purify, the community's desire for improvements to the game mode's balance in general prompted the developers to move up the schedule on some planned changes to the format. In a reddit post on September 6, 2016, Ben Brode explained that while the developers had plans in motion to improve the situation in the "medium-long term", the community discussion had made them "rethink our timelines and options". As a result, on September 8, 2016, Blizzard announced that an additional 45 cards would be removed from the Arena in an effort to rebalance the classes. This was by far the biggest set of exclusions to date, as well as the biggest change yet to the game mode as a whole. The developers explained that the goal of the exclusions was to reduce the power of mage and rogue - long the most successful classes in Arena - and to increase the power of all other classes, except for paladin, which was already at around the desired power level. When selecting cards, the developers tried not to exclude 'story cards', or to remove too much class identity.

Move to Standard and changes to card probabilities
In September 2016 the developers mentioned that they were planning changes to improve the balance of Arena play in the "medium-long term". This saw a number of specific card exclusions from Arena, which are documented above. A few months after the card exclusions, the developers commented that they were still "working on a more elegant system" to balance the game mode.

In February 2017 the most substantial changes to the format yet were announced. Most significantly (and controversially), the Arena was changed from Wild to Standard format. In addition, a number of specific rules modifications were announced to the base probability of cards showing up in drafts: Common, Basic, and neutral Classic cards would show up less in drafts, while spells would show up more often in drafts. Abyssal Enforcer and Flamestrike were also specifically given a 50% reduced chance of appearing in drafts. A less critical change was the long-awaited addition of golden cards to the format, something that had been requested since the game's beta.

With the release of the Journey to Un'Goro expansion on April 6, 2017, Quest cards followed in the footsteps of C'Thun cards in being banned from Arena due to their lack of sufficient synergy in non-constructed decks.

Micro-adjustments were first being discussed and tested in July 2017.

Starting from August 10, 2017, the hero cards from the Knights of the Frozen Throne expansion were initially available in the Arena upon launch,  but on November 14, 2017, they were removed from the draft pool since "while the Death Knight Hero Cards are exciting and powerful to play, their permanent Hero Power upgrades are hard to combat in a format where answers are limited".

Introduction of Arena card buckets
On March 6, 2018, Lead Systems Designer Kris Zierhut put out a Designer Insights video talking about the implementation of a revolutionary card bucket system for Arena in the near future, in Patch 10.4 on March 11, 2018. Cards of similar power levels would appear in each card selection screen during the Arena drafting process.

On May 31, 2018, a Developer Insights blog post was posted detailing how this new system worked. Arena card buckets, card weights, and micro-adjustments were explained in detail. Further improvements to the card bucket system were also discussed to be added in the next patch update, Patch 11.2, which was later implemented on June 5, 2018.

Cards were grouped together into card buckets based on their win rates and pick rates as seen in past Arena history, thus achieving the effect of grouping cards into buckets of similar power levels. Therefore, when a player picked cards during the Arena drafting process, they would no longer have easy "instant-picks" but instead have more meaningful cards of similar power levels to choose from.

The bucket system in its entirety is explained in the insightful: May 2018 blog post.

On August 1, 2018, another Developers Insights blog post came out discussing the problem with low card variety in the current card buckets system. Fixes were to be implemented to try to address this problem of Arena decks feeling too same-y in the upcoming patch, Patch 12.0, which was implemented on August 2.

Two card buckets lists were made public by Blizzard in and after this blog post, detailing the cards that appeared in each bucket power level:
 * card_buckets.csv - August 1, 2018
 * card_buckets.csv - December 19, 2018

Each card in Arena had its own chance of appearing in draft picks. An official appearance rate list was also made public in the same blog post:
 * appearance_rates.csv - August 1, 2018

On November 29, 2018, Tian Ding, a Senior Data Scientist with the Hearthstone team explained the math behind Arena balance in a Developer Insights blog post. Tian explained how the system tries to get class win rates as close to 50% as possible, using initial value estimates of card performance using past history of its class win rates. This is done through building a model, solving constrained optimization problems, and finally calculating weights for every Arena card. However, initial value estimates of card performance were not always perfect, and so final results of class win rates could deviate from the ideal value of 50%.

Tian Ding's blog post can be found here: Developer Insights: Arena Balance Through Science.

An Arena update was rolled out on December 19, 2018, where old cards which were banned were restored to Arena, and cards from the latest expansion Rastakhan's Rumble increased by 50% in appearance rate, attempting to vary the composition of Arena decks over the course of that expansion.

Appearance rates of cards continued to be adjusted to ensure the overall win rates of each class remained as close as possible to the ideal value of 50% with Patch 16.0.8 on January 9, 2019.

On August 16-17, 2019, Game Designer Dean "Iksar" Ayala posted multiple times on Reddit about the potential removal of the card bucket system from Arena in the near future, in an upcoming patch. Card buckets were later removed from Arena in that patch, Patch 15.4, on October 8, 2019.

As of August 26, 2019, the then-current Arena rules could be found in a forum post maintained by Community Manager Dylan Bates, named "Current Arena Rules". The forum post is no longer updated.

Move to Limited Wild and two month rotation
On April 9th 2019, with the start of the Year of the Dragon, Arena rotated its draft pool every 2 months, where players drafted cards from select Standard and Wild card sets to keep things fresh. Previously, Arena was played using a Standard-only draft pool.

All active Arena runs ended when the rotation changed. If a player had an active Arena run, they were given the appropriate reward for the number of wins they had, and also a ticket to allow them to start a new Arena run free of charge.

After the introduction of two-month Arena rotations, the micro-adjustment process noted in the section above now occurs typically within a month after a new Arena rotation has taken place.

In September 2019, cards shown in Arena selection screens were on average composed of around 79% Common, 15% Rare, 5% Epic, and 1% Legendary cards.

Return to Standard
On April 7th 2020, starting with the Year of the Phoenix, the Arena draft pool returned to Standard format, discontinuing the two-month set rotations used in the previous year and reestablishing the classic Arena system whereby the cards offered in Arena changed only with the release of new card sets.

Return to Limited Wild and two month rotation
On November 17th, 2020, with the release of Madness at the Darkmoon Faire, the Arena draft pool will return to the prior format of a curated selection of Standard and Wild sets.

Trivia

 * While the game client itself does not feature seasons for Arena play, some regions may offer special promotions rewarding performance in the Arena. On June 1, 2016, the official Chinese Hearthstone site announced that the players with the highest number of 12 win-runs each month would be eligible for special prizes, including tickets to . This was a Chinese initiative, but the game's developers intend to learn from the results of the experiment.
 * The symbols used for keys 4-10 are the same as the original Ranked play medals, in use for most of the game's closed beta. The first 3 keys are slight variations on the first 2 Ranked medals, although some of the keys have been given different names.
 * With Whispers of the Old Gods the legendary minion C'Thun and the 16 cards that offer specific synergy with it were the first new cards since the release of Elite Tauren Chieftain during the game's closed beta to be specifically excluded from the Arena. The 17 cards also represented the largest proportion of excluded cards until September 15, 2016 when an additional 45 cards were excluded.
 * Following the reveal of Purify from One Night in Karazhan, along with the video response regarding the card's design, it was also announced that the card will not be available in Arena, due to its limited synergy applications and possibly from community backlash.
 * On March 22, 2016, streamer TwoBiers became the first known Hearthstone player to successfully complete the 100 in 10 Arena Challenge.

The following card selection rules for Arena no longer apply or are no longer accurate:
 * Old card selection rules
 * From February 2017 to April 2019, only cards from Standard format were included in picks, except for during special Arena events.
 * As of Patch 9.0.0.20457, the first two sets of cards in a draft are more likely to include synergy-based cards. On September 9, 2017, synergy picks were removed from Arena.
 * In February 2017, 26 of the 30 picks choose a rarity according to the follow odds: 68% Common/Free, 20% Rare, 9% Epic, 3% Legendary.
 * The remaining 4 picks are 'Rare' picks that have a chance to be upgraded to Epic or Legendary at a slightly higher rate.
 * Any individual common card should show up about as often as any individual rare card.
 * In April 2019, cards shown in the Arena selection screens were on average composed of around 83% Common, 11% Rare, 5% Epic, and 1% Legendary cards.
 * Cards from the latest expansion or adventure are more likely to appear, at least for a while after the release of the related content. For example, during January 2016 cards from The League of Explorers were given an additional 50% likelihood of appearing. The duration of this boost appears to end prior to the next content release, with One Night in Karazhan seeing an increase from August 11 to October 20.

The following cards had a reduced likelihood of appearing in draft picks, on top of the other factors listed above.

Quotes
When selected at the start of a run, each hero will speak a unique soundbite, similar to an emote. These quotes are also heard in the Heroes tab of the Collection.

Patch changes

 * Far Watch Post, Mor'shan Watch Post, Crossroads Watch Post and Kargal Battlescar are excluded from Arena selection.
 * Fixed a bug where the Arena could fail to load properly for new players.
 * Bucket system is removed.
 * November 19, 2018 Update
 * The following cards are no longer excluded from the Arena selection process:
 * Druid: Savagery - Soul of the Forest - Mark of Nature
 * Hunter: Starving Buzzard - Timber Wolf - Snipe
 * Priest: Mind Blast - Lightwell - Inner Fire
 * Shaman: Dust Devil - Totemic Might - Ancestral Healing - Windspeaker
 * Warlock: Sacrificial Pact - Sense Demons - Succubus
 * Warrior: Warsong Commander - Charge - Rampage
 * Mind Control Tech can no longer be drafted.
 * Ancient Watcher, Humongous Razorleaf, Whizbang the Wonderful, and all Odd or Even-cost synergy cards remain undraftable.
 * Bonus appearance rate for the most recent set has returned.
 * Bonus appearance rate for the most recent set has returned.


 * Unknown patch (August 1, 2018)
 * Class Cards appear less often: +100% -> +50%
 * Spells and Weapons appear less often: +75% -> +50%
 * Appearance rate of Rare and Epic cards is lowered.


 * Unknown patch (May 31, 2018)
 * New card buckets added that overlap halfway between the existing buckets.
 * For example, a new bucket between buckets 2 and 3 will contain the weakest half of the cards from bucket 2 and the strongest half of the cards from bucket 3.
 * There will be 6 new buckets for non-Legendary cards (for a total of 13), and 2 new buckets for Legendary cards (for a total of 5).
 * The strongest cards in the best bucket and the weakest cards in the worst bucket no longer appear in the overlapping buckets, effectively reducing their rate of appearance.


 * Unknown hotfix (March 16, 2018)
 * Legendary picks now appear less often. Below average picks will now appear more often.


 * Unknown patch (November 14, 2017)
 * Death Knight Hero cards are removed from Arena.


 * Vicious Fledgling can no longer be drafted.
 * Removed synergy picks.
 * Small appearance rate adjustments.
 * Arena Rewards: Reduced variance of rewards in Arena. In particular, reward bags will no longer contain single, non-Golden Common cards.
 * Arena Draft Change – The first two sets of cards in a draft are now more likely to include synergy-based cards.
 * Creating decks for the Arena will now feature Standard cards only. Arena runs that were started prior to the patch will still include Wild cards.
 * Rares, Epics, and Legendaries will be offered more frequently during Arena deckbuilding.
 * Spells will now show up with a higher frequency.
 * Neutral Basic / Classic cards will appear less often.
 * If you own a Golden version of a card, the first copy you draft will be Golden. If you own multiple Golden copies of a card, all copies you draft will be Golden.
 * 45 further class cards have been excluded from the Arena selection process:
 * Druid: Savagery - Poison Seeds - Soul of the Forest - Mark of Nature - Tree of Life - Astral Communion
 * Hunter: Starving Buzzard - Call Pet - Timber Wolf - Cobra Shot - Lock and Load - Dart Trap - Snipe
 * Mage: Forgotten Torch - Snowchugger - Faceless Summoner
 * Paladin: No exclusions
 * Priest: Mind Blast - Shadowbomber - Lightwell - Power Word: Glory - Confuse - Convert - Inner Fire
 * Rogue: Goblin Auto-Barber - Undercity Valiant
 * Shaman: Vitality Totem - Dust Devil - Totemic Might - Ancestral Healing -Dunemaul Shaman - Windspeaker
 * Warlock: Anima Golem - Sacrificial Pact - Curse of Rafaam - Sense Demons - Void Crusher - Reliquary Seeker - Succubus
 * Warrior: Warsong Commander - Bolster - Charge - Bouncing Blade - Axe Flinger - Rampage - Ogre Warmaul
 * The guaranteed pack from Arena runs will now always be from the most recently released expansion. Additionally, you are slightly more likely to receive a second card pack from a different Standard set.
 * The sound effect when picking a card in Arena is now quieter, and varies in pitch.
 * Players will now have a chance to receive any of the current Hearthstone card packs (which include the Classic Hearthstone set, Goblins vs Gnomes, and The Grand Tournament) whenever a card pack is awarded, with a greater chance to receive The Grand Tournament card packs.
 * Achieving 12 wins in the Arena is now accompanied by a message from the Innkeeper: "Ooh! You have claimed the top arena prize, the Lightforge Key!".
 * Heroes are now golden in the Arena if the player has 500 wins in Ranked play with that hero.
 * Some heroes have new unique emotes when selected at the start of an arena run.
 * All card pack Arena rewards are now Goblins vs Gnomes card packs.
 * Arena rewards at 10, 11, and 12 wins no longer reward common non-golden cards.
 * The number of matches you can win in Arena has increased to 12 (up from 9).
 * Arena rewards have been restructured.
 * Unknown late alpha or early beta patch: Rewards of 5 Arcane Dust have been replaced with non-golden common cards.
 * 'The Forge' has been renamed 'Arena'. [Undocumented change.]
 * Players no longer keep the cards they select in the Arena, and admission now costs gold instead of card packs. [Undocumented change.]
 * The number of matches you can win in Arena has increased to 12 (up from 9).
 * Arena rewards have been restructured.
 * Unknown late alpha or early beta patch: Rewards of 5 Arcane Dust have been replaced with non-golden common cards.
 * 'The Forge' has been renamed 'Arena'. [Undocumented change.]
 * Players no longer keep the cards they select in the Arena, and admission now costs gold instead of card packs. [Undocumented change.]
 * 'The Forge' has been renamed 'Arena'. [Undocumented change.]
 * Players no longer keep the cards they select in the Arena, and admission now costs gold instead of card packs. [Undocumented change.]