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 certain cosmetic statues to be unlocked for the courtyard.
Notes[]
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:
'...The Underworld is the most ancient of the places in this world; the center and foundation on which all of surface-dwelling life came into being. It is a place composed chiefly of two primal forces: One of them is darkness. The other is heat. Heat is the binding force of this land, for within the earth resides a furnace hotter than the forges of Hephaestus. As darkness forms the shape and substance of the Underworld, heat forms its power -- a power harnessed by the Master and Lord of Death himself, in his eternal vigil to ensure his realm continues to be feared and insurmountable.'