They exist to destroy. The entire cosmos and everything in it is a means to an end, and that end is the extinction of existence itself -- all worlds stripped bare, or shattered. All that lives dead. All that can support life corrupted and befouled. Within the ranks of the Burning Legion are fiends who enjoy the corruption of mortal hearts, who savor the slow degradation of those who willingly sign the pact -- but in the end, the purpose of the Legion is the purpose of its master, Sargeras -- and that purpose is the unmaking of all things.
Feared and reviled on a host of worlds, the Legion has brought devastation everywhere it has alighted. Only one world ever survived a direct assault by the Legion, the poor, beleaguered, wartorn world of Azeroth. Twice now, the shadow of the Legion has fallen over this world of mortals, and yet, while far greater beings, god and demigods stood and fell against their might the world of Azeroth managed to find a way to balk the coming of the Burning Legion not once, but twice. For that singular distinction, Azeroth has earned the Legion's undying enmity. It's not a question of if the Legion will return. It's a question of when.
How did this nearly infinite host of nihilistic hatred and fel-touched fury arise? What are the hosts of the Legion and why do they march on this crusade to destroy all that exists?