Enemies to the last.

As the priest to Beast Master Zellas Metallium, one of Chaos Dragon Garv's equals, Xelloss is a powerful mazoku in his own right. As Zellas was opposed to Garv, so too was Xelloss and as a result, Xelloss has no love for Valgarv. The two hate each other and fight frequently throughout the series, with Xelloss being the only one who can really fight him on equal footing.

I love watching these two fight! Their battles give us insight into Xelloss' true nature as a mazoku and shows us Valgarv's true powers. Their fights are often very fast paced -- so much so that it's hard to actually see what's going on. However, when we do get to see them together, it's often very intense.

At some point, Xelloss receives orders from Zellas to recruit Valgarv into their ranks. He explains that Garv's destruction, followed by Phibrizo's destruction, has left the mazoku with a significant power gap. If Valgarv were to join the mazoku, it would fill the gap considerably and in return, Xelloss would kill Lina so that Valgarv could finally get his revenge. Valgarv of course declines, feeling that the mazoku have no right to try to recruit him after what they did to Garv. Xelloss admits that he felt that it would be pointless to ask but he couldn't disobey orders.

With Valgarv's refusal, Xelloss' orders change; if Valgarv would not join them, Xelloss would kill him instead. All too happy to be done with the formalities, Xelloss and Valgarve begin to fight. However, as Valgarv is still suffering from his arm retaining its ryuzoku form, he's easily overpowered. Xelloss pins him to the ground with his staff, twisting it cruelly into Valgarv's transformed arm and relishing in the torture and pain he's causing his opponent. He reminds Valgarv that it was he who killed thousands of ryuzoku in the Kouma War and that Valgarv shouldn't underestimate him.

This triggers some memories for Valgarv and as he manages to unleash a blast of energy that distracts Xelloss, he tells him that he shouldn't underestimate him. With ferocity, he attacks Xelloss with the Ragudo Mezehis, injuring him greatly with the light weapon. He reminds the mazoku that he has the power of both races and Xelloss on the ground, broken and helpless to pursue him any further.

Valgarv then proceeds to summon Dark Star and is believed to be dead after their encounter. When the time comes for Dark Star to be destroyed once and for all, we see that Valgarv somehow survived, being absorbed by the dark lord. Xelloss is more than happy to lend his mazoku powers to destroy him, orders or no, as the mazoku would prefer to destroy the world themselves than let it be done by an outside force.