A combo deck is a deck that revolves around executing one or more specific combinations of cards, to great effect. Combo decks may aim for one turn kills, slightly slower two or three turn kills, or simply powerful plays that allow the player to seize control of the board. However, most combos aim to win the match, either through direct damage or through establishing an overwhelming advantage, usually in one or two turns.
The exact definition of a combo deck is debatable, although any deck which relies on a specific combination of cards as its win condition can be reasonably called a combo deck. In terms of pacing, combo decks are often control decks, with the non-combo cards serving to delay the game until the key cards can be drawn, while others are mid-range decks that can hold their own without their combos, but with the right cards can quickly turn a balanced match into a rout. Aggro decks are rarely considered combo decks due to the independent value of their cards and speed of their playstyle, leaving little time for drawing the right cards, although they may still contain certain key card combinations.
Some combo decks feature a single specific combo, while others have a variety of synergistic options. Combos that are very limited or specific to only a few cards generally cannot be played in full until all the key cards are in hand, while combos that have wider synergy with other cards in the deck are more flexible and can be played at various points in the match. Players using control combo decks will often slowly collect the pieces of the combo over the course of the match, hoping to draw them all before they lose the match, while more versatile combo decks will often find various pieces of a potential combo fairly easily, but wait until the optimal situation arises in which to make their play.
Now defunct, the Combo Druid used from the game's release until Whispers of the Old Gods was an example of a fairly straightforward two-card combo deck. The deck used 's original effect - summoning three 2/2 Treants with Charge - in combination with to deal at least 14 damage in a single turn, plus another 2 damage for each other friendly minion. Combined with , sticky minions or (in combination with or ) a second Savage Roar, this combo could easily defeat players at apparently quite safe levels of Health, even from an empty board. However, the dependence on just four cards - two copies of each key card - meant the druid often failed to draw the combo in time, and would be forced to play out part of the combo prematurely in order to stay alive. Smart opponents could also play around the combo by keeping the druid's board clear, their Health high and/or a Taunt or two in place.
Patron Warrior is an example of a mid-range combo deck with a variety of synergies and only a loose win condition. The key combo card, , can be combined with various activators, effects which enable the combo to occur, or that generate value from the combo cards, such as , or . However, to get the best value out of the Grim Patron the warrior needs to activate the minion repeatedly, generating potentially unlimited value from repeated trading and Grim Patron summoning, allowing them to gain strong board control and set up for a later devastating attack on the enemy hero itself. Patron Warrior often also features , which if left unattended to can provide a one turn kill after activation through AoE damage such as Whirlwind, or . Other synergistic cards include and , which similarly all benefit from the same activators. This range of synergies makes for a number of possible combos, resulting in a strategically complex deck.
Victory with a combo deck is heavily dependent on drawing the right cards. For this reason combo decks frequently feature abundant card draw. This is especially critical for combo decks relying on just a few cards to win.
Combo decks are often significantly non-interactive, with the opponent unable to prevent the player from collecting the pieces needed for their devastating play. This is strongest in decks which use one turn kill combos, due to the suddenness of the defeat, and the sense of powerlessness in preventing it. Because of this, excessively powerful combo decks are often the subject of nerfs, in order to make the decks more interactive and less frustrating to play against.
The best defence against combo decks is usually to try to deny them their win conditions. For combo decks that rely on the state of the board or the survival of a certain key minion this is achievable, but in combo decks that require only drawing the right cards, there is little that can be done to specifically prevent them executing their combo. The main strategies against such decks are to maintain a high Health/Armor total, preventing them from achieving a one turn kill, the presence of Taunts or Immune effects (namely ) and/or simply to defeat them before they are able to collect the right cards. Knowing the exact amount of damage such decks are capable of outputting in a single turn is often critical to outplaying a combo deck, especially in the later rounds. However, combos do not need to kill the player outright to achieve a win, and the player should try to keep a plan for their own victory in mind beyond mere survival.
Combo decks are one of the most problematic deck types for designers attempting to provide a balanced meta. However, in the right proportions they can also serve to add a lot of variety to the laddering experience. Max McCall states:
- "Hearthstone is fun because each game is a little different from the last. Combo decks make for very different types of games, where players can’t rely on their normal decision-making heuristics and have to reconsider their strategic approach to the game. So, in general, combo decks are good for Hearthstone because they add some texture to the ladder experience."
However, McCall states that combo decks can become problematic when they become too popular in the meta, especially since they tend to be relatively non-interactive. This can lead to frustration for the opponent, especially when the deck features one turn kill combos. Less powerful (or especially complex) combo decks tend to be less popular, and thus don't tend to become a problem for the health of the game, with unusual combo decks in particular serving to add a lot of variety to the meta.