Update 25/07/2024:
https://huggingface.co/PGR-RVC/NieR_RVC_v2/resolve/main/EN/FatherNier_e260_s14040_RVCv2_RMVPE.zip?download=true
The Shadowlord is actually the original Nier, who was the player character in the beginning of the game. In the year 2053, 50 years after the incident where the Dragon and the Giant appeared in Tokyo, Nier and Yonah (as real humans) were trapped in Shinjuku when its lockdown commenced. Nier sought help for Yonah's growing disease from a charity organization's center that supposedly handed out food and medicine for the survivors. Instead he, along with twelve other people, received a clone of the Grimoire Noir. The charity was actually a human experimentation site designed to find the "Original Gestalt", who can keep their own consciousness even after going through the Gestalt process.
Original Gestalt is the name given to Nier as the first Gestaltified individual to retain control over his own ego. It became necessary for the Original Gestalt to continually supply his maso to prevent other Gestalts from relapsing and to help sustain their ego. Additionally, Gestalt Nier entered into an agreement with the World Purification Commission to continue providing his maso for more than 1000 years in order to save Yonah, who had already begun to relapse. In the end, with no word on the project's success centuries after the projected date, Gestalt Nier lost faith in the promise and gave into loneliness. He kidnapped Replicant Yonah to force Gestalt Yonah into her body. This set to motion the Replicant Nier's quest for vengeance.
Just like the Replicant Nier (the character played throughout most of the game), Shadowlord's ultimate goal is to save his daughter and humanity (the Shades). All the sentient Shades are linked to the Shadowlord, and his death would cause them to relapse.
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huggingface.co/PGR-RVC