Also known as: ソウルオブニル
The "Essence of Chaos", Void Termina's core.
The boss that takes this name in particular is exclusive of the Soul Melter difficulty in "The Ultimate Choice", replacing a nameless version of Void Termina's core from the main story.
Void Soul themself is also replaced in Soul Melter EX (the highest difficulty) by Void (Kirby)