Burning Legion

From wow.incgamers.com

The Burning Legion the name given to the army of demons by their leader, the fallen Titan, Sargeras. The Legion is a force of darkness on many worlds of the Twisting Nether and the Great Dark Beyond composed mainly (only?) of demons. Sargeras' goal is the utter and total annihilation of the universe, to end existance as it is imperfect and should be undone. Abandoning the benevolent Pantheon of the Titans, Sargeras smashed the prisons he had constructed and forged an army of demonic entities with himself as the leader.

He chose found the peaceful, but very potent Eredar and corrupted two of the three leaders into servitude as demons; Kil'jaeden the Deceiver, who drew the Dreadlords into his service and chose from among their ranks Tichondrius as his second, and Archimonde the Defiler, who rallied the brutal Pit Lords to him and chose Mannoroth as his servant. Dubbing his malefic force the Burning Legion, Sargeras and his army set about their mission to undo all the works of the Titans. The remaining Eredar that was not corrupted by Sargeras' might fled through the Twisting Nether and made their home on a world they called "Draenor", and themselves the Draenei - The Exiles.

First drawn to Azeroth by the Night Elves' delving into the magic of the Well of Eternity, the Burning Legion has returned several times in an effort to burn the world to nothingness on behalf of their unholy crusade. The Legion inflicted the Orcs with demonic blood, increasing their blood rage, almost exterminating the Draenei in the process. It is clear that the Legion has not yet been defeated, but simply staved off for the time being. However, Sargeras' minions are still a constant trouble for the Alliance and the Horde in Outland as well as on Azeroth.