The Possessor is a minor villain in the video game; Kingdom Hearts II, but makes a cameo in Kingdom Hearts 358/2 Days. It's a Pureblood Heartless that's rather weak, but is able to take control and animate the various statues and sculptures around the castle.
Kingdom Hearts 358/2 Days
Roxas experiences a memory of Sora fighting an army of Possessors while he was in the Rabbit Hole of Wonderland while he was on one of his missions. However Sora didn't fight any Possessors until later on, which shows the connection between Sora and Roxas revealing itself.
Kingdom Hearts II
A Possessor appeared in front of Sora as he was about to enter the dungeon doors. It ended up possessing the Dungeon doors in the Undercroft, transforming them into creature known as 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 and his friends were able to vanquish the Possessor, returning the Thresholder 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 ended up disappearing without attacking Sora.
Another Heartless known as the Dark Thorn had the ability to summon Possessors to throw at Sora, but these were destroyed as soon as they are thrown.