Exhausting a card puts it in your Exhausted Cards pile. Cards in this pile are effectively removed from combat and cannot be used again. If you exhaust a card that was in your deck, it will be returned to your deck at end of combat.
There are three main ways a card can become Exhausted:
- Some cards exhaust themselves when used (Like Impervious). These cards normally have a much more powerful effect than others at the same cost, but can only be used once. Some of these will no longer exhaust on use once upgraded. (Like Calculated Gamble)
- Some cards exhaust other cards as part of their effect (Like Burning Pact). Although this may seem like a downside, being able to remove cards you don't need can be an upside, especially if you have received a Curse or are fighting an enemy that can create Status cards.
- Some cards have a keyword called Ethereal (Like Carnage). Cards with Ethereal keyword will exhaust if they are in your hand at the end of the turn. If you discard them or use them before the end of the turn, they will not exhaust.
As of Weekly Patch 12, the only way to re-use a card that has been exhausted is through use of the card Exhume.