Xellos was a Mazoku that is several thousand years old at the least. He goes by the title of Mysterious Priest. Serves Beastmaster Zellus Metalium, and is quite powerful. Nearly killed all the golden dragons singlehandedly in the War of Monsters Fall. His favorite phrase is "That is a secret!" Xellos is constantly manipulating situations to his own liking, even if that means betraying friends and allies.