Xavius was changed into the first satyr by Sargeras and has been said to serve him in the Legion expansion. Yet, he is ruler of a realm created by Yogg-Saron, housed Ill’gynoth the Herald of N’zoth, employed the forces of the faceless, and corrupted his surroundings rather than destroyed them as the Legion would.
So, what master does Xavius serve?
N’zoth! He has all along.
No joke.
Himself. He was a selfish, arrogant douc**.
Xal’atath pointed out that Xavius seems to change paymasters to suit himself (and more specifically suit his desire for revenge upon Malfurion) though really he hasn’t directly served the Legion since the WotA, and since the events of Stormrage it seems like he and the satyr as a whole are pretty firmly Team Old Gods now.
One supposes that could change again if he returns (or among the remaining satyr), though their swap of loyalties seemed to at least partly be thematically related to the night elves losing their role as the world’s go-to “anti-Burning Legion” race to the draenei, so the races of corrupted night elves have been systematically shifted away from serving the Legion into serving the Old Gods or Void instead.
He will never serve!
In a large 50s style diner that was rather incongruous with the aesthetic of the rest of Azerothian architecture (outside of Kezan anyway) a goblin cook set a completed breakfast plate on the counter and ran the little dinging bell that sat beside it. “Xavius! Order up, table three, got Two Dots an a Dash, Checkerboard with Machine Oil, with a Hot Blonde in Sand ta drink!”
Xavius sneered. “I will NEVER serve!..”
The goblin cooked raised and eyebrow and removed the cigar that was otherwise perpetually in his mouth. “You want the gold you’ve being paid, you’d better serve, and serve whoever damn well makes any order the damn well please. Jeez, that attitude!.. it’s no freakin’ wonder you don’t get tipped well!..”
Old Gods. He hopped on board with the Legion during the War of the Ancients because they were a destabilizing force. The Old Gods saw the war as a opportunity to slip their cages in the chaos - it’s the same reason that they pulled the trigger on Deathwing’s betrayal at the same time.
No one now. He’s dead. Last I heard Malfurion was using his bones as chew toys.