Bosses

Bosses are extremely difficult monsters that are fought at the end of each act. Compared to normal enemies and elites, bosses have significantly more health and a more powerful move set that usually involves special gimmicks that cripple the player or heavily buffs itself. Every boss has a unique portrait shown at the top of the Spire map.

The map location prior to a boss fight will always be a Rest Site, allowing the player to heal before the fight. The only exception to this is the Corrupt Heart in Act 4 which always has an elite encounter containing the Spire Shield and Spire Spear.

Ascension Modifiers

Ascension⁴: Bosses deal more damage.
Ascension⁹: Bosses have more health and will sometimes gain more Block.
Ascension¹⁹: Bosses have more challenging movesets and abilities.
Ascension²⁰: Two bosses are fought in a row at the end of Act 3.

Rewards

Upon defeating the Act 1 and Act 2 bosses, the player is rewarded with Gold, a choice between three Rare Cards, a choice between three Boss Relics, and sometimes a random Potion. The player is healed for 100% (75%⁵) of their missing health at the end of every boss battle.

The Act 3 and Act 4 bosses do not reward any consumables or cards.

Killing a boss will add to a run's Score. Killing it without taking any damage, not including self-damaging cards like Bloodletting, adds another 50 points. Killing all three bosses without taking any damage adds 200 points.

Interactions

Relics

Pantograph: Heals for 25 HP at the start of a boss encounter.
Slaver's Collar: Gain an extra Energy at the start of each Boss and Elite encounter.

Wing Boots cannot be used to skip boss fights, as all paths in the map will lead to the boss.

Other

During the Mind Bloom event, choosing [I Am War] starts a fight with a random Act 1 boss for a random Rare Relic. This fight does not add towards the run's Score, but will activate the above relics as normal. This fight will not count towards the achievement Perfect.

To fight the Corrupt Heart, the player must kill the Act 3 boss with the Ironclad, Silent, and Defect at least once. This means the Heart cannot be fought on the first playthrough of any character except for the Watcher.