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 Master Ava and told Ventus to meet her in an abandoned house. The Darkness possessed him and killed Strelitzia when she arrived and took her rulebook. Upon leaving Ventus's body, it took the form of Master Ava once more, allowing Ventus to believe he was being chosen as one of the new 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.