Possessor
The Possessor is a boss from Kingdom Hearts II.
The Possessor worked with the Thresholder's Heartless to defeat Sora in Beast's Castle. After being forced out of its body, it floated about helplessly before returning to the door, bringing the Thresholder back to life. Sora could not defeat the Thresholder until the Possessor's was drained of life force, after which it disintegrated.
Along with the Thresholder, the Possessor is the first Heartless boss in the game.
Other Appearances
If the player looks closely when they defeat a Gargoyle Knight or Gargoyle Soldier, 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.
