Xellos is a mazoku and the priest of Greater Beast Zelas Metallium.
Xellos was created by Zelas during the Kouma War. The mazoku put a barrier over the peninsula and managed keep the enemy's reinforcements out of the battle. Because of this, however, four of the five lords were unable to participate in the war anymore, so priests and generals were created in order to represent them. Because he is her only immediate servant, and he was created from a great deal of her power, Xellos is more powerful than all the other priests and generals of Ruby Eye Shabranigdu's subordinates.
In the Kouma War, Xellos single-handedly killed thousands of golden dragons; he is feared and hated by them ever since. They refer to him as the Dragon Slayer, although he does not like this name, probably because it does not fit the "image" he wants others to view him as. After the war, before serving Hellmaster, Xelloss's duty for a thousand years was to find stray copies of the Claire Bible and destroy them so that humans could not find an easy way to defeat mazoku (with Zanaffar), or utilise the power of the Lord of Nightmares.
Xelloss' true astral form is that of a simple black cone. He is able to divide his power into dozens of tiny cones that can inflict damage over a long period of time.
