You walk in on Adria and some other magisters doing a ritual to re-create the soul stone. But you are too late and they finish and you gotta kill Adria and her possie. Once you do that the dungeon starts shaking and Diablo spawns or something. Then the encounter starts.
Hmm................very intriguing. Rather than Atria, I would say the last of the Horadrim, they would try to use Diablo's power to restore their power, but would become corrupted in the process, and would become minions, and bodies for Diablo to inhabit.