Diablo will free Inarius from where Mephisto has kept him for all these years. Diablo will use Belial's powers to mask himself as someone that Inarius would trust. His freedom will be awarded in exchange for his willingness to bring back Lilith to Sanctuary so they can use the nephalem to destroy heaven and hell. Throughout the story, you will be following a storyline that makes you walk the line between hero and villain. In the end, your character sacrfices themselves to stop diablo a "final time." Unfortunately, the power you release to kill diablo just separates the evils from Diablo as the Prime Evil, which is what Lilith was planning the whole time.
Flow: 1. Diablo releases Inarius.
2. Inarius works to release Lilith.
3. Hero kills Inarius but it's too late, Lilith returns.
4. Lilith hides and build nephalem army using a worldstone artifact. Much of heaven and hell are destroyed.
5. hero finds lilith after battling through the army and kills her.
6. Diablo reveals himself and uses Lilith's knowledge to give himself the power of the nephalem. The hero sacrfifices him/herself to 'kill' diablo.
7. Nephalem's (the hero) powers rip prime evil diablo apart and release the remainder of the prime/lesser evils (as well as diablo).