Grimoire Noir is a magical tome which like Grimoire Weiss uses magical spells through ‘words’. He accompanies and helps the Shadowlord.
In the 21st century a human experimentation called ‘Project Gestalt’ designed to find the ‘original Gestalt’ who could keep his own consciousness after going through the ‘Gestalt process’ used survivors from an incident in Tokyo. Fourteen people among them Nier and Yonah were given a clone of ‘Grimoire Noir’ that caused the survivors to turn into relapsed ‘Gestalts’. After seeing the other survivors turn into relapsed ‘Gestalts’ Nier and Yonah escaped only to be attacked by the relapsed ‘Gestalts’ forcing Nier to seek help from ‘Grimoire Noir’ so he could fight them off. When Yonah accidentally touched ‘Grimoire Noir’ it caused her to relapse.
Grimoire Noir is the Gestalt of a Hamelin Organization Crusader (a group of child soldiers raised to use magic to fight against the legions created by the ‘White Chlorination Syndrome’ that spread from the Grotesqueries Queen) that was bound into a book. The true purpose of Grimoire Noir was to fuse with Grimoire Weiss to force the Gestalts (Shades) to return into their Replicant bodies and become true humans again. Although he is turned down by Grimoire Weiss who is repulsed by the idea.