The Shadow Stalker was summoned by Xaldin after he failed to turn the Beast into a Heartless. While Sora and his friends were speaking with the Beast, the Shadow Stalker chased down Belle to the ballroom, where she called for the Beast's help. Sora, the Beast, and the others arrived just in time so Belle could escape from the Heartless. The Shadow Stalker then turned it's attention to Sora and his friend, and proceeds to fight them. Similar to the Possessor that resided below, the Shadow Stalker possessed the room itself, using the windows, pillars, chandelier, and floor to attack Sora and the Beast.
However, after Sora dealt enough damage to the Shadow Stalker, the chains on the Heartless' body began to fall off it. This causes the Shadow Stalker to mutate and transform into the monstrous Dark Thorn. In this new, powerful bestial form, it attacks Sora and his friends ferociously with furious attacks and its ability to become invisible. However, Sora was able to use the very elements that the Heartless had recently possessed to beat it into submission. The Heartless fades back into darkness, forcing Xaldin to retreat and reform his plan.