Sacrifice

Sacrifice is one of the five Endings

Achieve it by activating all 5 of the Monoliths, offering them each a magic item (any item with a blue border)

Entering the underworld through an archway once all of the monoliths are activated.

Defeating the dark spirits that guard blue flames in the underworld

Sacrificing yourself to the Demon King.

This unlocks the class Demonblood