We talk about the great evils of the Warcraft universe - your Kil'jaeden's and Archimonde's, your Sargeras and your Old Gods - and of course there's the mortal monsters like Gul'dan, quite possibly the single most evil mortal being ever to live, or Kel'Thuzad who enslaved himself to evil so profound that his transformation into a lich was probably a palliative influence on his malevolence. But what of evil as a corrupting vector? What of an evil so profound that it is part of your blood, that can seep into the souls of an entire race? Ladies and gentlemen, I ask you - what of Mannoroth, king of the Pit Lords, master of the Annihilan?
We know little of the Annihilan. They are the pit lords, a massive and powerful species of demons recruited to serve the Legion by none other than Kil'jaeden himself. They were already a powerful part of the Legion when the War of the Ancients unfolded - Mannoroth himself was one of the Legion's elite who came through the portal to help make clear the way for Sargeras to stride forth into Azeroth, doing battle with the forces of the Kaldorei who were attempting to preserve their lives. But as to where the pit lords come from, it is as yet a mystery unsolved. What world did they originate on? What happened to it? We do not know.
What we do know is this - few races in all the cosmos can boast so powerful and corrupting an evil nature as the Annihilan. And Mannoroth was for eons first among the pit lords - the acknowledged master of his kind, the most powerful of them all. Not Brutallus, not Magtheridon, not Azgalor, but Mannoroth ruled the Annihilan on behalf of the Legion.