Xyburn

Xyburn was a doomlord of the Burning Legion who participated in the War of the Ancients and made it his personal mission to eradicate the tauren race. The tauren hero and leader Huln Highmountain swore to kill him, and the two bitter foes finally clashed during the battle for the Well of Eternity near the end of the war. The fight ended only when Huln, seemingly beaten, landed a near-killing strike to Xyburn's face with his spear. The demon narrowly deflected the weapon, but since the spear was imbued with spirit powers it nonetheless struck Xyburn in the left eye, blinding him on that side. Huln won the battle, but before he could slay the doomlord, Xyburn was expelled back to the Twisting Nether along with most of the demons in central Kalimdor. In the Nether, the doomlord licked his wounds and awaited the Legion's next campaign.

His current whereabouts are unknown.

Speculation
"Doomlord" or "doom lord" has been used both to refer to powerful ered'ruin (such as doomguard) and to the demonic species which Krosus is a member of. It is unclear which of these two races Xyburn belonged to.