Ritual

Defeat Nightmare King Grimm. After beating Grand Performance and collecting the last round of Grimmkin flames, return to the tent to face the Nightmare King - a far faster, harder version with fireball walls, a triple-dash and a vicious second phase. One of the toughest bosses in the game; expect to learn it over many attempts. Mutually exclusive with Banishment - completing the Ritual keeps Grimmchild and locks out banishing, so do that on a separate save.