The Burning Legion (aka The Legion, Burning Shadow, great burning shadow, or Legion of the Lower Planes) is a vast, innumerable army of demons, infernals, and corrupted races who seek to destroy any trace of order in the universe. They are led by the fallen Titan Sargeras. The Legion has thrice invaded Azeroth, and were only beaten back at a terrible cost. Much of Azeroth's history — including the First, Second, and Third War — were due to the Legion's influence. It can be said that the Legion is Azeroth's nemesis; it corrupted the orcs who then invaded Azeroth, created the Lich King and the Scourge, and is the source of most of Azeroth's scars. It should also be noted that Azeroth is the only known planet that they have ever failed to conquer.
The Legion's mission is to utterly destroy the universe and remake it without the underlying defect preventing true order. In practice, the Legion scours all life from the worlds it invades, they are cursed to roam the greater realms forever to consume the primal energies of magic to survive.
The Legion has ravaged world after world in its mad lust for arcane power and is responsible for the annihilation of countless species.
The Burning Legion has attempted to destroy Azeroth three times: during the War of the Ancients, the Third War, and the War in the Broken Isles.