I think I know the story:
Tyreal, destroying the Worldstone and going against the Heavens to aid humanity, while under oath will be banished for his actions.
Out of anger and spite, he will keep his oath, enduring a transformation, from Angel to Demon he will attempt to set the world of Sanctuary into war once again.
Our heroes will arise to fight the challenge, both new and old, eventually finding out about Tyreal's actions. Tyreal would attempt to stop what he brought into the world of Sanctuary, but fail.
For some unknown reason, you will have to slay Tyreal in his new Demon Form, once released he will tell you about the full extent of his plans. He resurrected Diablo and Mephisto with shards of their soulstones, aswell as recovering a significant part of Baal, (Perhaps a skull or a spike?) resurrecting Baal aswell.
Though these three would be too weak on their own to pose enough of a threat to win Tyreal back his seat in the High Heavens upon their defeat, Tyreal infuses them with powerful beasts, or eachother. There, their powers grow out of control and out of Tyreal's control. Then, you must face the Demon(s) alone, as Tyreal has fallen to shame.
The original plan:
Even Tyreal has personality defects. One of which would involve reckless, childish behavior. Being cast out of Heaven for his acts, he would spite them, masterminding the return of the Three in some form or another and claiming that it was their plan after destroying them utterly.