At the end of Diablo II, the soulstones of both Diablo and Mephisto were destroyed at the Hellforge. It's been a couple of years since I've played the game, but if I remember correctly, it was explained that this would forever destroy the essence of the two Prime Evils. (I'm pretty sure Baal's soulstone was also destroyed at the end of the expansion.)
I just reached Act IV of Diablo III, however, and it becomes obvious that Diablo was less than vanquished.
Do they ever explain how this was the case? In particular, if his essence was never completely destroyed, how did it come to be entrapped within the Black Soulstone (along with the essences of the other five Great Evils destroyed in Diablo II)?
Also, I would prefer it answers to this question would not include spoilers for Act IV (and beyond).