Possessor

The Possessor (ポセッサー) is a Pureblood Heartless that is found in Kingdom Hearts II. It is one of the bosses at Beast's Castle. The Possessor is quite weak on its own, but is able to animate the various statues and sculptures around the castle.

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.

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. It is unleashed once the Thresholder is defeated, and hence it is very vulnerable to attacks as it flies out and back. When it exits and enters the Thresholder it is considered a wave and a phase of the boss battle.

Other Possessors appear when the Gargoyle Knight and Gargoyle Warrior Heartless are destroyed, but disappear without attacking Sora.

Trivia

 * Possessor has been shown to appear in Beast's Castle and Twilight Town 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.