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. But unfortunately the Darkness entity possessed him and shortly after one of the new union leaders Strelitzia arrived looking for Player and killed her for unknown reasons. It commanded Ventus to take her rulebook and upon leaving it took the form of Master Ava once more causing him to take her place under the lie that he was one of the new 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.