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.

Design
The Shadow Stalker's body is a black sphere with several twisted tentacles sprouting out of it. Two of these are large, each sporting two metal bands around them, while the rest are smaller and placed more haphazardly around the Heartless's body. The tips of all of these tentacles are orange. The Shadow Stalker's jagged maw is red on the inside, and its eyes are eerie, glowing, yellow circles with pitch-black rings around them. The Most prominent feature of the Heartless are the two chains wrapped around its body, with each end of both chains attached to a conical weight. The Shadow Stalker is vaguely reminiscent of the Darkball Heartless. The Dark Thorn is a massive creature with black skin. Its entire body is wrapped in teal thorns, possibly a reference to the Beast's rose, and both of its wrists and ankles still have manacles on them. The chains on its arms are broken, but the chain connecting its feet is not. The Heartless has a large, red, toothy maw and beady, glowing yellow eyes. While it is muscular and has large hands and feet, it also has rather thin limbs. Each of its limbs sports three digits with long, red claws. The Dark Thorn also has two enormous, orange horns and a massive, orange mane.

The Shadow Stalker's name refers to its status as a Heartless, a Heart corrupted by the Darkness. A "stalker" can be defined as someone who secretly follows another, often with ill intent. This may reference the fact that the Shadow Stalker was sent to torment the Beast. The Dark Thorn's name also references the Darkness, and "Thorn" harkens back to the thorns covering its body. Its Japanese name, "Black Thorn", holds a similar meaning, though "Black" could also be referring to its own dark coloration.

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.
 * The Dark Thorn's chest has a heart-shaped hole in it, almost identical to that of the Darkside. This is a trait shared by all Pureblood Heartless bosses.