Diablo 1 established him as an evil or something. I barely remember the story. Lord of Evil/Darkness sort of story. Diablo 2 sets up the Prime evils. Mephisto, Diablo, and Baal. Hatred, Terror, and Destruction respectively. Diablo 2 also establishes that the person to kill diablo at the end of the game was the warrior, and he was corrupted by the mere pressence of diablo, causing him to think only he can contain the soulstones power by jabbing it into his own body. Diablo 3 I think changed it to Greater/Lesser evils. Belial, Azmodan, Andariel, and Duriel. All I remember is one is "Lord of Sin." It also changed the Warrior who killed diablo into Prince Leoric. And all the heroes from Diablo 2 have gone insane due to muh corruption. There was a war between the High Heavens and hell till a demon and angel decided not to fight, I think. Kinda like Preacher. They gave birth to the humans/nephalem. The angels and demons saw their power and felt threatened so they tried to kill them. With the use of the world stone, it protected the humans and nephalem from the Angels/Demons. Their power reduced, their minds wiped. The 7 evils (3 greater, 7 lesser) invaded sanctuary, and ran amok or someshit. This is all book shit and I haven't read the books. Tyriel, an angel, destroys the world stone revealing sanctuary to all, and awakening the powers of the nephalem. I'm going off halg memories, so I have no idea what I am missing, but it is kinda schlocky.