If you did the Warlock Green Fire quest you know that the Doomguard referred to his his people as the hounds of the titans,forever forced to police the arcane, before Sargeras freed them. The Doomguards or Ered'ruin were obviously used by the titans, maybe even created by them. This not only means that the Titans are even more directly responsible for the creation of the Burning Legion than we thougt they were, but it made me think, what if the Legion has control over Titan construct machines?
I mean they conquered so many titanic worlds, they must have found at least one of their machines, that they now use to produce demons 24/7, after all the Doomguards seem to know something about the Titans, but even if not the Legion must have found one of those machines. And when those demons die, they just go back to the nether until they regenerate, essentially the legion's numbers increase forever. I cant wait for more revelations by Blizzard.