There are several rumors and theories surrounding the origin of the Xelloss we know today. I am aware of to of these such backgrounds. One of the stories supports the idea that Xelloss originally was human and was befriended and made a mazoku by Zelas. The other is that Zelas created him to suit her purposes. Xelloss fought in the War of the Monster's Fall, and was single-handedly responsible for what nearly brought the complete obliteration of the golden dragons. Xelloss is Zellas' priest and general rolled into one, and is said to currently reside in Wolf Pack Island. Xelloss' past is vaugeand not entirly known, however more is becoming of Xelloss' history all the time, and with more Slayers to come, I'm sure we'll learn more and more about Xelloss with every installment.