Diablo is an action role-playing game franchise owned and produced by Blizzard Entertainment. , the series has sold over 18.5 million copies worldwide.
Diablo is set in the world of Sanctuary. In the universe there is an eternal conflict between the High Heavens and the Burning Hells. The High Heavens are led by the Angiris Council and the Burning Hells are led by the Prime Evils. At an unspecified point in time the Arch-Angel Inarius grew weary of the never-ending struggle, and, gathering a handful of like-minded angelic companions, left the battlefields. While searching for a place untouched by battle, Inarius and his followers encountered a group of Demons from the Burning Hells who had also grown weary of conflict. Led by Lilith, the daughter of Mephisto (eldest of the Prime Evils), these demons joined with the angels and, over time, intermingling caused attraction. For the first time, there were unions of angels and demons. Even Lilith and Inarius fell in love. Eventually, by gradually manipulating the very substance of the universe, Inarius created the world of Sanctuary. He also created a massive crystal-like object known as the "Worldstone," which he bound to Sanctuary and to himself. He used it to hide Sanctuary from the ever-roving eyes of the High Heavens and Burning Hells.
Over time, the union of angel and demon blood created beings with astounding powers, named "Nephalem". When Inarius saw their astounding power he grew worried that they would disrupt "his" world and planned to destroy them. But the other angels and demons pleaded with him to not destroy their children, and so he held off for a short while. However, Lilith wanted to raise the Nephalem to become her servants and make herself ruler of Sanctuary, and went about murdering the other angels and demons. For this she was banished by Inarius and he slaughtered most of the Nephalem and manipulated the Worldstone to lessen the chances of the Nephalem's power developing. This progressed almost to the point of Inarius devolving them. Lilith came back to continue her plot to use the Nephalem as an army, but was stopped by a farmer named Uldyssian-ul-Diomed, who rose against her and Inarius. He destroyed both of their cults and sacrificed himself to protect the world. This caught the attention of the Archangel Tyrael. He established the Horadrim, who were to prevent the lords of the Burning Hells from ever taking over the world. The Horadrim captured the three prime evils, first to be captured was Mephisto, Lord of Hatred, then Baal, Lord of Destruction, and finally Diablo, Lord of Terror.
The prime evils remained imprisoned until Diablo was able to reach some of the mortals who lived in the town above him, called Tristram, and started bringing minions from Hell into Sanctuary. A hero arrived and managed to slay him, and tried to contain Diablo's soul within himself. He was unsuccessful and fell under the demon's influence. With this warrior as a new host, Diablo set about to free his brothers and eventually took over the warrior's body, assuming his true form. Another band of heroes went after him and managed to slay Mephisto and then Diablo. Baal continued north and proceeded to the Worldstone. Here the band of heroes fought and eventually slew him, but not before he had a chance to corrupt the Worldstone. Tyrael arrived and sent the heroes away to safety, then destroyed the Worldstone, causing consequences that even he could not foresee.
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Official Site: Blizzard Entertainment
Links: Wiki on diablo and his brothers Diablo bio and stats Diablo II Database