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The Annihilan are a powerful species of demon recruited by Kil'jaeden to serve the Burning Legion. Their leader, the most feared of them all, was Mannoroth the Destructor.
The Pit Lords who serve under Mannoroth the Destructor are some of the most cruel, barbarous butchers to ever roam the trackless wastes of the Twisting Nether. These hulking engines of hate and death love only to kill and bring sorrow to all living creatures. Fanatically loyal to Mannoroth, the Pit Lords will stop at nothing to see the will of the Legion upheld. These massive Demons take sadistic pleasure from instilling crazed terror, and then slaughtering their enemies.
At some unspecified point in time, Kil'jaeden recruited the annihilan into the Burning Legion, an invitation the pit lords readily accepted. This would have occurred somewhere between twenty-five and eleven thousand years ago; the pit lords were a major integrated part of the Legion by the War of the Ancients ten thousand years ago. Annihilan are rarely seen on the battlefield, since they are too important to simply waste in combat. The only two annihilan that entered Azeroth during the War of the Ancients and the Third War were Mannoroth and Azgalor, so it is presumed that the annihilan are either generals for smaller forces of the Burning Legion or are still on the Legion's homeworld.
Mannoroth's blood was used to forge a blood-pact with the corrupted orcs on Draenor, making their corruption permanent — only one clan, the Frostwolves, escaped. During the Burning Legion's second invasion, Thrall and Grom Hellscream defeated Mannoroth, and broke the blood pact he had forged and thus freed the Orcs from demonic corruption. Hellscream did not survive the encounter.
Though the blood of both Mannoroth and Magtheridon has been used to corrupt the orcs, it is not clear whether this is an ability inherent to all Annihilan, or simply to certain powerful individuals. Since there are few pit lords willing to submit to experimentation, it's unlikely the question will be answered anytime soon.
The annihilan pit lords are described as engines of destruction. They are driven by pure rage and bloodlust. Their bodies resemble massive lizards, with six powerful limbs (four legs, similar in nature to the hybrid races, such as the centaur and the dryads), a tail, and wings sprouting from their backs (the wings are vestigial and occasionally used for defense — even with eredar magic, pit lords are far too heavy to fly). Their upper torsos are more humanoid, with a large chest, two arms and a large horned head. Their mouths are filled with rows of teeth, their eyes are on fire and even their tongues are studded with fangs. The preferred weapon of the pit lords is a massive double-bladed sword. Though they may look fat and lazy, they are unimaginably powerful. Some annihilan are depicted with beards.
Nothing is known of the culture of the annihilan, but it is likely one where raw power is valued above all else. While Mannoroth was certainly quite powerful, he probably rose to become their leader through his cunning and magical knowledge — he is said to have been as skilled as a mid-level eredar. Besides Magtheridon, no other pit lord has shown particular affinity for the arcane arts, though many of them know the 'fel firestorm' spell.