Diablo

When the world of Sanctuary was created, it was a realm blessed by the light and damned by the darkness. It was a world were the Nephalem could reside out of the conflicts of the Celestial Wars. The world and the Nephalem were created by the Archangel Inarius and the Demon Lilith. When Lilith saw the potential of the Nephalem she began creating an army to tip the scale of balance. Inarius hoping to prevent this from occuring used the Worldstone to diminish the power of the Nephalem, thus upon each successive generation becoming normal humans.

When the Heavenly Council was made aware of Sanctuary, they tasked the Archangel Tyreal to destroy it.
However, after realizing the potential and worthy souls he petitioned for mercy. Thus Sanctuary would be watched, but the minions of hell had other plans. As the centuries rolled on, demon lords began to take interest in Lilith's creation... each one tried slipping into the world. Yet time and time again, they were thwarted thanks to the unseen hand of the Archangels. Things stayed this way for centuries until a new order
came to play in hell. Three powerful demon lords calling themselves the Prime Evils (Baal, Diablo and Mephisto) toppled Lucifer from his position. While in power they invaded and attack Sanctuary physically. With such a bold move, Tyreal put together a society of mages known as the Horadrim to combat the Prime Evils. They managed to defeat and imprison them. Other Demon Lords soon took the Prime Evils place and renewed the process of small battles here and there on Sanctuary

However, evil festers, and soon the Prime Evil broke free. Diablo was the first to break his chains by corrupting those around him. Although he would be defeated by a hero, he would soon find have a host to help free his brother. He managed to free Baal from Tal'Rasha's prison and reunited with Mephisto. Before focusing on Sanctuary, they sent Diablo to deal with things in hell. Unfortunately though, Mephisto and Diablo were defeated by a group of mortal who tracked the Wanderer from Tristam. Baal had escaped, but made his own planes to destroy Sanctuary by massing an attack on the Worldstone. Thanks to Tyreal, his plans were stopped but only after much devastation.