Xenocrates Bastier is a Possessor Demon, and the most powerful of his kind. Able to enter and control individuals at will, he is a formidable foe. He wishes to gain enough power to overthrow the Master of the Underworld, and will use and manipulate anyone in order to achieve this goal. Recently, Xenocrates dominated a clan of imps by forging an alliance with their Imp Master, Valdamar. However, Valdamar secretly wished to vanquish him and restore himself to power, and tried to steal a Book of Shadows in order to obtain enough power. Xenocrates, after a failed attempt at stealing a witch-vampire Jay Anthony David Robert's powers, he was contained in a crypt and taken to Klaustrum Prison.
It is impossible to vanquish Xenocrates while he is is possession of an actual, physical body. However, his essence can be contained using magic.