Juu-oh Zelas Metallium created Xelloss more than one thousand years ago. Not long after Xelloss' creation, the Kouma War occurred. During the battle, Xelloss managed to single-handedly murder thousands of golden ryuzoku, nearly wiping out their entire race. He did this with the use of only one finger. To this day, those of the golden ryuzoku who recall the events in the Kouma War are frightened of Xelloss and try to avoid him at all costs.
When the Kouma War was over, Xelloss was ordered to destroy copies of the Claire Bible that contained information on how to create Zanaffar. If humans learned the secrets to creating many Zanaffar, the mazoku would be in danger of extinction. We do not know how long Xelloss was destroying these Claire Bible manuscripts or what other tasks he was assigned from the time between the Kouma War to when he meets Lina Inverse.
Hundreds of years passed and while Xelloss was destroying copies of the Claire Bible, Mei-oh Phibrizo borrowed him. Xelloss was given new orders to follow... To follow Lina Inverse and make sure that no harm had come to her until it was time for Phibrizo to use her in his plot. Phibrizo, like all other mazoku, wanted to destroy the world and return to the Sea of Chaos, to the Lord of Nightmares. To do this, Phibrizo planned on forcing Lina to cast the Giga Slave and then kill her so the spell would become uncontrolled.
Xelloss followed Lina around and protected her from various dangers when she became powerless. The threat of Maryuu-oh Gaav was getting closer to Lina but Phibrizo himself stepped in and ruined Gaav. When Phibrizo was later ruined by the Lord of Nightmares, Xelloss returned to Juu-oh's service.
An unknown amount of time passes when Lina receives a request for help from a young woman addressed as Filia Ul Copt. As they set out on their adventure, Xelloss sets out on his own. He has new orders now: to recruit Valgarv into the ranks of the mazoku. The demise of Mei-oh Phibrizo was a devastating blow to the strength of the mazoku and Valgarv would be a welcome power. However, Valgarv refuses, remembering how the mazoku betrayed his mentor. Thus, Xelloss' orders change once more to ruin Valgarv.
As time progresses, Xelloss soon finds himself in the same situation as Lina. The mazoku are unhappy at Dark Star's arrival and do not wish to see the world destroyed by hands that are not their own. Xelloss is sent to steal Galveria from Lina and he keeps it in safe keeping until it is time to destroy Dark Star. When the time comes, Xelloss combines his powers with Lina and Filia's, aiding in the destruction of Dark Star. As Slayers TRY ends, we see Xelloss reporting to his Lord... what orders are in store for him, we have yet to see...