Mephisto and Baal Join the Party
At the bidding of Decakrd Cain, the last living Horadrim, a group of adventurers was tasked with pursuing Aidan/Diablo and preventing the release of his brothers. This group included a young barbarian, who is stated by the developers to be the very same barbarian that is playable in his older, grizzled form in Diablo III. Unfortunately, they arrived too late at the crypt where Baal was imprisoned. He had been freed from his chains and taken full possession of Tal Rasha. The reunited Lords of Hell were two steps ahead of the heroes, well on their way to freeing Mephisto (above).
When at last the mortal champions arrived at Mephisto's resting place, they found him already free. He had stayed behind as a rear guard while Diablo returned to Hell to reclaim it from the Lesser Evils, and Baal had set out to secure the Worldstone. Tapping into their Nephalem heritage, they were able to topple Mephisto and seal him away again. From there, they had only one way to go: straight into Hell to pursue Diablo.
That only left one loose end...
- tl;dr: A group of heroes fought their way into Hell to destroy Diablo and Mephisto for good. Well, supposedly for good...
Showdown at the Worldstone
The heroes arrived at Mount Arreat, the resting place of the Worldstone, to find its brave, barbarian defenders beaten and Baal's forces laying siege to their last line of defense. They fought their way through the lines of demons, but by the treachery of one of the barbarian elders, Baal had already bypassed the ramparts and reached the Worldstone. They eventually found their way to him and defeated him, but discovered that his ritual of corruption had already begun.
Tyrael arrived just in time, and used his angelic sword to destroy the Worldstone rather than let it become permanently tainted. The energy released destroyed Mount Arreat, Baal, and Tyrael. How at least one of the mortal heroes survived (the barbarian, who appears again as a playable character in Diablo III) is a mystery.
For a time, the conflict between the mortals of Sanctuary and the Burning Hells had ended. For a time...
- tl;dr: The heroes confronted Baal at Mount Arreat. Archangel Tyrael blew up the Worldstone and the entire mountain around it to keep the stone from becoming corrupted. Baal is dead, and Tyrael is missing. Heaven is taking notice.
Evil is Back in Style
Now you're all caught up to the “present day” in the Diablo universe. It has been 20 years since the destruction of the Worldstone, and Deckard Cain believes the remaining Lords of Hell -- Azmodan and Belial -- are plotting to bring about the end of days. A star has fallen on the Tristram cathedral, the former resting place of Diablo. Whisperings among mystics of something called the Black Soulstone are beginning to circulate.
Perhaps most disturbing of all, with Tyrael gone, the forces of Heaven may judge Sanctuary too dangerous to continue existing. It seems it will fall again to a group of mortal heroes to decide the fates of Heaven and Hell.
- tl;dr: The bad guys are coming to kill us all. Best be prepared.