Thresholder

The Thresholder (ゲートガーディアン) is a Heartless boss from Kingdom Hearts II.

A giant, two-headed, four-armed Heartless with even bigger arms and hands, it could extend from one side the Undercroft to the other. It is accompanied by Gargoyle Knights, Gargoyle Warriors, and Hook Bats during the battle phase. In actuality, the Thresholder is simply the Possessor possessing the Undercroft dungeon doors of Beast's Castle, which Sora unlocks out of the door twice to defeat.

If Sora returns to the Undercroft after defeating the Thresholder, Gargoyle Knights and Gargoyle Warriors will appear. It is one of the few bosses that King Mickey will appear in if Sora dies. It is one of the Heartless placed by an Organization XIII member known as Xaldin, who tried to turn the Beast into a Heartless with hopes of obtaining his heart and his Nobody.

Kingdom Hearts II
The doors possessed by Possessor that attacked Sora and his friends.

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

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. Try to avoid his hands and breath when attacking him. 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.

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 Thresholder. Sora learns Upper Slash and Guard, Donald learns Fire and Goofy gain a health boost.

Video
Video:Kingdom Hearts 2: Thresholder & Possessor Battle

Trivia
The carved doors which later become the Thresholder Heartless make a cameo appearance in Kingdom Hearts 358/2 Days.