Shadow Stalker and Dark Thorn

The Shadow Stalker and its stronger form, the Dark Thorn, is a Pureblood Heartless that is found in Kingdom Hearts II. It is one of the bosses at Beast's Castle.

Kingdom Hearts II
A Heartless that attacked Belle in Beast's Castle.
 * Shadow Stalker

It jumped around on the floor, pillars, and chandelier of the ballroom, creating mayhem.

A liberated version of Shadow Stalker that appeared in Beast's Castle.
 * Dark Thorn

It appeared and disappeared while running wild, creating a chaotic battle for Sora and his friends.

Kingdom Hearts II
The Shadow Stalker is a Pureblood Heartless summoned by Xaldin after his first attempt to turn the Beast into a Heartless failed. While Sora, Donald, and Goofy were speaking with the Beast, the Shadow Stalker chased Belle down to the ballroom, where she called for the Beast's help. Sora, Donald, Goofy, and the Beast arrived just in time to see Belle escape the Heartless out onto the balcony. The Shadow Stalker then turned from her to the party. Similar to the Possessor Heartless that resided below, the Shadow Stalker possessed the room itself, using the windows, pillars, chandelier, and floor to attack Sora and the Beast.

However, when it is driven from the walls, its chains are released and it transforms into the monstrous Dark Thorn. In this new, powerful bestial form, it attacks the party ferociously with furious attacks and its ability to become invisible. Sora, however, is able to use the very elements that the Heartless had recently possessed to beat it into submission. The Heartless fades back into darkness, and Xaldin leaves the castle to reform his plan.

While the Beast and Belle reunite after the attack, Sora speaks briefly with the servants of the castle, then opens the gateway to the next world.

Shadow Stalker
The Shadow Stalker will possess the various parts of the ballroom during the encounter to attack the party;
 * Windows: The Shadow Stalker flies into the windows, and causes a giant flash to pass over the floor. Retreat behind the pillars to avoid this.
 * Floor: When the Shadow Stalker disappears into the floor, it will attempt to devour Sora from beneath. Jump sideways when the floor underneath him glows to avoid the attack.
 * Chandelier: The enemy Heartless will fly up in the air, and bring the chandelier crashing down to the ground, triggering a shockwave. The chandelier will turn, firing energy beams. Attack the chandelier to release the Shadow Stalker. It has 59 HP.
 * Pillar: The Shadow Stalker will fly into the pillars, which it will cause to flail and thrash about. Attack them to release the Shadow Stalker. It has 39 HP.

Once you have depleted the HP of the chandelier or pillars, the Release reaction command will appear. The Shadow Stalker will fly out of the object it was possessing, releasing Drive and MP Orbs. Take this opportunity to bombard the enemy with attacks, or unleash a Limit if you feel you can survive MP Charge, or have an Ether handy. This process will continue until victory is obtained.

Dark Thorn

 * Attacks
 * Claw Attack - Damage x1
 * Dark Servant (Dark attack) - Damage x1
 * Swing Around - Swing Damage x0 / Throw Damage x1.5
 * Chandelier Attack - Fall Damage x1.5 / Spin Damage x2
 * Throws you at pillar - Unknown
 * Becomes Invisible

This boss battle would be relatively easy if it weren't directly after the Shadow Stalker (giving no time to save and recover in between). This battle requires noticeably less strategy than the Shadow Stalker and more in the way of a straightforward assault.

Despite the fact that it turns invisible, it is still able to be targeted - this being a boon to players during the battle. However, it is more agile when invisible and the fight becomes much easier when the invisibility is lost following the use of the Stepvault Reaction Command, during which Sora will jump onto the chandelier, and, following another command, lower the chandelier and swing it about in circles, hitting the Dark Thorn and rendering it visible. Following this somewhat-dramatic assault, the battle becomes considerably easier, as the boss moves more slowly. The Dark Thorn is also able to summon Possessors to attack Sora.

Trivia

 * According to Nomura, the Dark Thorn's design is based off of Ifrit, specifically the Final Fantasy X version.
 * If knocked out in this battle, it is possible to play as King Mickey.
 * The Shadow Stalker bears an uncanny resemblance to the Darkball Heartless. The only differences is that it has chains, extra "arms" sticking out, and it has a smoother outer covering.