Asmodeus

Asmodeus is the 13th and final ghost of the Chest of Demons. He is also the main antagonist for the film Scooby-Doo! and the Curse of the 13th Ghost. However that version of Asmodeus turned out to be just another guy in a mask. By the end of the film it's left open ended as to what became of Asmodeus.

History
He was originally an ancestor of Vincent Van Ghoul until he went mad with power and had to be sealed within the Chest of Demons. It was in the chest he supposedly became a demon.

Trivia

 * It is unknown whether he was redeemed or whether Velma lied to Vincent Van Ghoul about his ancestor wanting redemption as Shaggy points out what they saw was just smoke. If Velma is incorrect it mean Asmodeus is still out there hiding in the world.