Here's what I understand from the history of the soulstones. (there is a great overview
here.
- The Soulstones were (possibly) cut from the WorldStone by Tyrael (not the Prime Evils)
- The Worldstone was used to suppress the powers of humans (who, from what I understand, are the descendants of angel/demon cross-breeds?)
Regardless, there is some power inherent in the Soulstones. This power is what the Prime Evils were after.
- Tyrael forged the Soulstones to use against the Prime Evils
- Izual told the Prime Evils how to corrupt the Soulstones and use their power to bridge the worlds of Hell and Sanctuary (for an invasion)
- The 3 Prime Evils arranged their own banishment from Hell to Sanctuary
- Like a sucker, Tyrael used the Soulstones to imprison the Prime Evils
- The Prime Evils corrupted their Soulstones and used that power to eventually respawn in Sanctuary and open that gate between Hell and Sanctuary (invasion time!)
- Diablo went to gather Hell's armies, but before he could finish, he and Mephisto got thwomped (sending their spirits back into their soulstones) and their Soulstones were crushed, theoretically closing that bridge, isolating Sanctuary, and sending the Prime Evils to some sort of abyss...
A few questions remain for me:
1) (obvious) What happened to Baal's soulstone?
2) (obvious) What effects on Sanctuary will shattering the Worldstone have?
2a) Will humans regain the powers subdued by the Worldstone?
2b) Will Sanctuary become visible to the legions of Heaven and Hell?
3) To where were Mephisto and Diablo banished?
3 becomes important, because if they were simply banished to Hell, the destruction of the Worldstone makes Sanctuary available to them anyway.