The Anti Black Coat is a being that emerged from the darkness surrounding Sora when he was plunged into sleep.
Kingdom Hearts 3D: Dream Drop Distance
Riku arrives in the ruined Castle That Never Was and reaches Sora, contained in a floating orb swirling with darkness; images of Riku, Kairi and Pluto float throughout the orb. Riku attempts to pull Sora free, telling him to wake up and not to chase the dream because if he did he'd delve too far into the darkness and never wake up, though this proves to be futile.
The darkness of the orb begins to separate from it forming the Anti Black Coat. Riku asks if it was the one who trapped Sora in his nightmares. The Anti Black Coat doesn't respond and Riku summons his Keyblade, while the Nightmare symbol manifests of the back of the Anti Black Coat, contrasting Riku's Spirit symbol, Riku states that if it was a Nightmare, he would defeat it, before engaging the Anti Black Coat in battle. After emerging victorious, the Anti Black Coat rises off the ground, and its darkness disperses.
The Anti Black Coat wears a standard black coat, which protects against darkness, however on the ends of the sleeves, as well as around the base of the coat, are purple flame-like hues. It manifests a blue Nightmare Dream Eater Symbol on his back, mirroring Riku's Spirit Dream Eater Symbol, before their battle. The Anti Black Coat's face is shrouded by the hood of the black coat, though two distinct circular red Nightmare-like eyes shine clear from under the hood.
Role in the series
In the spongebob series, the Anti Black Coat reveals itself to be a splintered piece of Vanitas's soul that was created by the Unversed Commander to try and stall for time while he manipulated Roxas into giving Sora enough pain in order for Xehanort to corrupt him fully to the dark side. The Anti Black Coat is currently unclear to get an appearence.