Darkness (character)

The Darkness is a mysterious character introduced in Kingdom Hearts Union χ.

Kingdom Hearts Union χ
Before the Dandelions left for the new world, an unknown entity called Darkness took the form of Ava and told Ventus to meet her in an abandoned house. Darkness possessed him and killed Strelitzia when she arrived and took her rulebook. Upon leaving Ventus's body, it took the form of Ava once more, allowing Ventus to believe he was being chosen as one of the new Union leaders.

Hearing Maleficent's frustration about not being able to rewrite her ending in her world, Darkness told her that she is trapped here because the witch knew too much information, which could mess with the timeline, and that she was trapped in a data version of her world. Soon, it directs her to Daybreak Town, when in the Clock Tower the Darkness takes her into a secret room called "The Ark", informing the witch that she could use one of the pods to escape to the real world.

Afterwards, when Maleficent asks if she made it to her own time, the Darkness answers she did not as the first pod was used to help her travel from world to world and to get to her own time she'd have to travel using another pod. Aware that Darkness had no physical form, the witch offered for it to come with her, but the entity turned her down being aware of unfinished business.

At some point in time, it returned to Ventus, but upon feeling its host go unconscious, the Darkness made itself known to the other Union leaders.

Kingdom Hearts III Re Mind
When Sora begins his journey to restore Kairi's heart using the power of waking, he first enters Ventus's heart. When attempting to wake Ventus, he is stopped by an entity referring to itself as "Darkness". Sora questions if "Darkness" is Vanitas based on the latter's last words, but does not get a reply as the entity forms itself into the Dark Inferno χ and battles Sora.