Nightmare’s Heart

""The expanse of dream in past was split, One realm now must stay apart, Darkest reaches, beating red, Terror of sleep. The Nightmare's Heart.""

- Seer

The Nightmare's Heart is an NPC in Hollow Knight. It has no dialogue but is mentioned by multiple other NPCs.

Lore
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.

Grimmchild and Troupe Master Grimm are its spawn. The Ritual performed by the Grimm Troupe is designed to sustain the Nightmare's Heart with a new vessel once the old one dies.

In-game events
The Nightmare's Heart is present in the background of the Nightmare King Grimm fight both in Dirtmouth and in Godhome. Its eyes glow red when Nightmare King Grimm reaches 50% of his health.