Possessor

The Possessor is a Pureblood Heartless that is found in Kingdom Hearts II and Kingdom Hearts 358/2 Days. 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. In this manner it can become the Thresholder, as well as a Gargoyle Knight or Warrior.

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

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

The doors possessed by Possessor that attacked Sora and his friends.
 * Thresholder

Possessor was separated from the doors with Release, in which the power of the Keyblade was poured into the keyhole blocked by its arms.

Kingdom Hearts II
The Possessor possessed the Dungeon doors in the Undercroft, transforming them into the Thresholder in order to fight Sora. After being forced out of the doors, it floats about helplessly before returning to them, reanimating Thresholder. Though it summoned many Hook Bats, Gargoyle Knights, and Gargoyle Warriors to assist it, Sora, Donald, and Goofy were able to vanquish the Possessor, returning the Tresholder into a normal door as the Possessor faded into darkness.

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

Strategy
When you first enter the Undercroft, you will encounter Gargoyle Knights. After they are defeated, move towards the Thresholder. Hook Bats will arrive to defend it. After you defeat a second wave of Hook Bats you are clear to attack Thresholder. The Thresholder will attack by extending its arms across the room and summoning orbs of darkness, so try to avoid those. When his health depletes to a certain point, you can now use the unlock reaction command to release Possessor from the door. Possessor has no form of attack whatsoever, so you're free to go on the offensive.

Eventually it will float back to the door and reanimate it. Thresholder then starts to shoot projectiles at you. Move in close to attack Thresholder without being hit. After you deplete its health again, Possessor comes out and floats around again. When it's health is gone, you have defeated both bosses.

Trivia

 * The Possessor/Thresholder boss battle is one of the few in which Mickey can appear.
 * 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.