Possessor

The Possessor is a boss from Kingdom Hearts II.

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

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

Journal Entry
''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
If the player looks closely when they defeat a Gargoyle Knight or Gargoyle Warrior, they may notice a dark wisp rising from the statue when it is defeated. This too is a Possessor (although not as powerful as the Thresholder; it dies immediately after being released). A reaction command named Release can be used on these enemies as well, and instantly kills them. Additionally, enemies such as the Hydra use an attack that makes small wisps rise from the ground and attack Sora. Although it is likely that these are not Heartless (as they are used by normal monsters such as the Hydra), they bear a resemblance to the Possessor, including the eyes.