Heimdall

Heimdall is the Sub-Boss of Annwfn (C-5). You cannot fight it until you have connected the Corridor of Blood after collecting all of the Dissonance.

To start the battle, use the Djed Pillar to chant the "End" Mantra (Nótt Mantra) in front of The Six-Sided Star of Eg-Lana in the background of the room.