So I was just reading up a bit on Xavius and came across this:
"Malfurion sensed however, that while Xavius was strong, he was merely the servant of something even more powerful. The true master of the Emerald Nightmare has thus yet to be revealed."
So is it possible that Blizzard have added a "hidden" boss (not in the dungeon journal)? Or are they just referring to Sargeras as Xavius serves the Legion?
Source: http://wowwiki.wikia.com/wiki/Xavius