Darkest reaches, beating red, Terror of sleep. The Nightmare's Heart.
The Nightmare's Heart is an NPC in Hollow Knight. It has no dialogue but is mentioned by multiple other NPCs.
The Nightmare's Heart is a higher being responsible for the Ritual of the Grimm Troupe. It is the deepest power within the Nightmare Realm, and also the one that controls the entirety of the Grimm Troupe.
↑ 2.02.1White Lady on the completed Grimmchild: "Success then for the scarlet heart, and irony, to use my spawn to grow its own. I know you creature, and the form time shall bring. You may be all and one, clan and master,"