He's a Character of Five Nights at Sonic's: Fierce Forces.

Created from the mind and memory of Silent "Player-On!" once she accepted a new rules in Fierce Forces game.

Abyss Crystalline King Boo has a simple mission.
Mainly he makes a difficulty for Luigi "Superstellar-Up!" of helping Silent "Player-On!" in some tasks.
He can trap the Green Brother in his Painting (And yes, Silent (all her versions)
can free Luigi "Superstellar-Up!", by using the Darklight Beam installed in her Flashlight)
Also, the spooky ruler of the limpid chasm has some other abilities, but that's a secret for now.

Edit: I had to correct his hands.