Quote from Darcy_MarisThen you (yeah, you) destroyed Diablo and thus the Black Soulstone. Or so I assume. There is no reference to you destroying it but none is there any reference to it being still in once piece....
I wouldn't know where you'd have to go, but I'm pretty sure Blizzard personnel have come forth and explicitly clarified that the BS is still intact. (You can see it falling through the clouds or whatever if you look close) Just put in my CE Behind the Scenes DVD, here is the Storyboard showing the soulstone is definitely still in one piece after Diablo disintegrated.
"But the Black Soulstone is flawed and cannot contain that many demons for long. We must capture Azmodan within it and shatter it... lest all these Evils be reborn."
As for Tathamet coming back, remember that this game was codenamed 'Project Hydra.'