Heat

Heat represents the level of self-imposed difficulty of Zagreus' escape attempts. The Heat level can be raised or lowered by using the Pact of Punishment before starting a run. Having a certain amount of Heat active allows Infernal Gates containing greater rewards to be unlocked. Escaping the Underworld on certain heat levels also allows statues of Skelly to be unlocked, which are purely cosmetic.

Trivia
During the initial part of Early Access, Heat was an obtainable item like Darkness or Ambrosia. As such, it had a Codex entry in the "Artifacts" section, which is now no longer accessible. The Codex entry read: