If I understand it correctly... [SPOILERS]
Tyrael takes 20 years to sober up after destroying the Worldstone in D2X. Then he comes to Sanctuary as the fallen star (and as a human ala James Remar's Raiden) to tell folks about the evil Belial taking over Caldeum in disguise these last 20 years.
Adria the witch from D1 gets you to trap Belial in the Black Soulstone which (?) already contains the souls of the 5 Great Evils defeated in D2.
Azmodan, the last Great Evil then invades Sanctuary through the weakness at Mount Arreat where the Worldstone used to be in order to get the Black Soulstone and rule with the power of the other 6 evils who are now in it.
After you kill Azmodan, Diablo possesses Leah (from inside the stone, with Adria's help) using the 7 evils' powers to become the 'Prime Evil' himself. Maybe this was his plan all the way back from the Dark Exile but it's set up really really poorly.
Diablo then goes to Heaven and kicks everyone's !@# until you go there and kill him.
The End. Did I miss anything?
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