Possessor

The Possessor is a boss from Kingdom Hearts II.

The Possessor possessed the Dungeon doors, turning them into Thresholder to fight Sora in Beast's Castle. After being forced out of the doors, it floats about helplessly before returning to them, bringing Thresholder back to life. Sora cannot defeat Thresholder until the Possessor is destroyed.

Along with Thresholder, the Possessor is the first Heartless boss in the game.

Kingdom Hearts II
''A Heartless that appeared in Beast's Castle. It possessed the dungeon doors, turning it into Thresholder.''

It had the power to conceal itself, but had very few other powers.

Other Appearances
Upon destroying a Gargoyle Knight or Gargoyle Warrior, black wisps of smoke can be seen within the broken rubble. Additionally, there is a reaction command called Release, which instantly kills these enemies, and a Possessor Heartless is seen to emerge from them. This could very well suggest that those two Heartless are also controlled by Possessors.

Trivia
Posessor has been shown to appear in Beast's Castle in Kingdom Hearts 358/2 Days. It floats around without possessing anything, similar to after the Release Reaction Command when it is used against Thresholder.