The Shadowlord is a shade and seems to be the leader of the shades or at least ranks very highly amongst them. He was originally a man named Nier from the early 21st century whose ‘Gestalt’ (soul) later came to be known as the Shadowlord. His daughter is the ‘Gestalt’ of Yonah and like Nier he desperately tries to provide the best for his daughter and to stop her relapse. The Shadowlord is accompanied and helped by Grimoire Noir.
In the 21st century Nier and Yonah became trapped in the Shinjuku district when its lock down commenced after an incident where Grotesqueries Queen and the Red Dragon Angelus appeared in Tokyo. He sought help from a charity organization that supposedly handed out food and medicine to survivors but instead along with twelve other people they received a clone of ‘Grimoire Noir’ and became part of a human experimentation designed to find the ‘original Gestalt’ who could keep his own consciousness after going through the ‘Gestalt process’. After seeing the other survivors turn into relapsed ‘Gestalts’ he 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.
As the first sentient ‘Gestalt’ Nier now the Shadowlord was offered by the organization behind ‘Project Gestalt’ to put ‘Gestalt’ Yonah into hibernation until she could return to her ‘Replicant’ body but in return he had to help them create stabilized ‘Gestalts’. Because of this all sentient shades are linked to the Shadowlord and his death causes them to relapse.
Realizing Yonah’s relapse couldn’t be stopped the Shadowlord kidnapped ‘Replicant’ Yonah in an attempt to reunite his daughter with her body setting in motion ‘Replicant’ Nier’s quest for vengeance.