The Diablo games are very different from anything in the Warcraft setting, a far more black-and-white morality played out in endless cosmic war between absolute forces of order and righteousness opposing evil and chaos. This war, the Great or Eternal Conflict, is so massive in scale and scope that it can almost be said to be endless. The war predates the world of Sanctuary, the realm where mortal men and women are born and live out their lives. Indeed, Sanctuary itself was created by a rogue angel and his demonic lover and ally as a place to find an end to the war.
But before the conflict, before the universe itself existed, there was only Anu. Anu existed as a pearl, a central point which contained all evil and all good, all possibilities and all dualities contained within its mirrored surface. Yet Anu felt imperfect in its duality and sought purity in the rejection of its negative aspects, casting them aside to become a being of glittering diamond -- a perfected warrior. Yet what Anu rejected, his cast-off impurities themselves became perfection of a dark bent, forming the monstrous Tathamet. This seven-headed dragon declared war on the glittering Anu, who in turn sought to destroy that which he had cast out of himself.