Fatigue

Fatigue is a game mechanic that deals increasing damage to players who have already drawn all of the cards in their deck, whenever they attempt to draw another card.

Mechanics
Each player starts the game with a full deck of 30 cards. All cards drawn are drawn from that deck, slowly depleting its supply of cards. The player's hero will announce when the deck has only one card left, and again once the deck is fully exhausted.

Once a player has exhausted their deck, trying to draw a further card will cause Fatigue, dealing damage to the hero. This includes cards automatically drawn at the start of your turn, and cards specifically drawn as a result of other card draw effects. Fatigue takes effect each time a card is attempted to be drawn.

Fatigue deals 1 damage to the hero, plus 1 damage for each time Fatigue has already dealt damage to the player. Fatigue therefore deals damage cumulatively, steadily increasing in power each time it deals damage.

Fatigue only deals damage when the attempt to draw a card fails. If the player restocks their deck, such as through shuffle into deck effects, the player will not suffer fatigue when then successfully drawing a card. However, Fatigue never resets. This means that even if the player restocks their deck, or successfully draws cards, the next time they fail to draw a card, Fatigue will pick up where it left off, dealing 1 more damage than it did the last time.

Because Fatigue is caused by the attempt to draw a card, not the movement of the card itself, similar effects such as put into hand effects, put into battlefield effects, discard effects and remove from deck effects will not trigger Fatigue.

Previous bugs

 * From Patch 3.1.0.10357 (2015-09-29) until Patch 4.0.0.10833 (LoE patch), a bug caused replacing your hero (such as by using Lord Jaraxxus or Majordomo Executus) to reset your Fatigue, so the next Fatigue damages for 1 again.

Strategy
Some control decks may employ Fatigue damage as a secondary or even primary means of winning games. Mage decks excel at this approach, thanks to multiple board-wide Freeze effects, delaying secrets like Ice Block, and a large range of removal spells. Players can also force the opponent to overdraw through effects such as Acolyte of Pain and Coldlight Oracle, while keeping their own card draw low. Effects like Alexstrasza's Battlecry can be useful for late damage when having focused on stalling until that point, allowing Fatigue to push the opponent over the edge.

Trivia

 * At one point during Hearthstone's development, the Fatigue mechanic did not exist. Instead, much like in many traditional card games, the player would instantly lose the game as soon as they ran out of cards. During one point in playtesting, the Hearthstone team asked (currently the game director on the Overwatch team) to come over and try the game. Kaplan, who had never tried the game before, "got really into it" and had a lot of fun figuring out what cards to play next. However, when he was very close to winning, he ran out of cards and instantly lost. This made it feel like none of the decisions he had made during the game mattered, since he was just going to lose as soon as he ran out of cards anyway. It was such a stark moment that the Hearthstone team said to themselves "Hey, after watching Kaplan have that experience, we need a different solution for running out of cards" and designed the Fatigue mechanic. Fatigue's gradually increasing damage "connects back in with the whole rest of the game" and makes it feel like every decision made and every point of damage inflicted matters. It allows players to know that the game will end at some point, but the moment when a player runs out of cards doesn't come as as much of a surprise and doesn't break them out of the gameplay experience the same way the previous "instant loss" mechanic did.
 * This effect is based on, a mechanic from World of Warcraft. Fatigue causes characters traveling through or over very deep water to become exhausted, eventually suffering substantial damage every two seconds. Fatigue is usually only incurred when attempting to cross oceans or swim between continents, and prevents players from swimming between the major continents, bypassing normal travel requirements, or reaching areas that are intended to be inaccessible, such as . Fatigue can usually be avoided by traveling by land or shallower waters; in some cases players may skirt fatigue in order to shorten their travel, but move back into shallower waters before they suffer fatal damage. In a few cases fatigue cannot be avoided, but players with sufficiently fast flying mounts are able to travel through the fatigue zone before they succumb to its effects. In these cases, fatigue acts as a limited barrier to travel between certain areas.
 * The amount of fatigue damage caused by each draw can climb at least as high as 14745.