Ember () is the resource used to play units and spells. At the start of each turn, your Ember is fully restored.
Each card has a corresponding Ember cost found on the top-left of the card (usually around 1-3, but cards such as the Consumer of Crowns cost 10). Some cards, such as the Automatic Railspike, cost . Playing these cards will consume all your remaining Ember (even if you have none) and their effects vary depending on how much Ember was consumed.
Each run starts with 3 base Ember, which can be increased with Artifacts such as Fel's Remorse. Cards such as Pyre Chomper can generate temporary Ember, while cards like Engine Upgrade can generate more Ember per turn for the rest of the battle.
The maximum you can have at any one time is 99.