Xelloss Metallium is both priest and general to Xellass Metallium, who is Beastmaster and one of five generals under Lord Ruby-Eye. He is the strongest out of all the servants under the dark generals. This is because his rank as both general and priest of Beastmaster gives him the shared power of a general and of a priest, instead of just one or the other. Even Chaos Dragon Gaav notes his power, saying that no one outside of Lord Ruby-Eye's five trusted companions has as much power as Xelloss.

       When Xelloss first shows up in the series, his assignment seems to be to destroy all copies of the Claire Bible. His true assignment, however, is to watch Lina and make sure she isn't killed and to watch for Gaav. He was on loan to Hellmaster Phibrizzo until he was killed. Hellmaster's ultimate goal was to force Lina to cast the Giga Slave and reduce the world to chaos. The Giga Slave spell, when cast incorrectly, returns the world to chaos, and the goal of all Mazoku is to reduce the world to such. Though Xelloss doesn't step into many battles (he prefers just screwing with the minds of his traveling companions), when he does he shows incredible strength.

       No one suspects that Xelloss is a Mazoku until about the last third of Slayer NEXT, when Lina figures it out. However, Xelloss had left several clues that he was one.

       In TRY he is the only person (other than Lina) to use Galbadia, the strongest of the five weapons created for the purpose of destroying Dark Star. He and Filia don't get along very well, because Filia is a dragon (gold, at that) and Xelloss is a Mazoku. Another person he doesn't get along with very well is Zelgadiss, but while Zelgadiss takes his anger out on Xelloss by non violent means (name calling, mostly. Those who've seen the two interact a lot would know what I'm talking about), Filia takes out her anger on Xelloss by whapping him several times with a large spiked mace.

       Xelloss carries a staff with him that has a red gem set into it. There are rumors going around online that the gem is a dragon's heart he obtained during the Kouma War. I wouldn't know if this is true or not because it could say that in the manga somewhere and my knowledge is based off the anime. Anyways, he uses the staff for two main purposes. One is as a weapon. He uses it as a weapon quite often, stabbing, choking and whapping people with it. Another way he uses it is as a channeler for spells, such as the Blast Bomb he cast while he still had those spiffy Demon Blood talismans. He was forced to sell Lina the talismans during NEXT.