Worldstone

The Worldstone was created by Inarius at the same time as when Sanctuary was first created. Its exact nature and properties is unknown to most, and not many know its full potential. Yet it is known that Inarius created the Worldstone to keep Sanctuary hidden from both the High Heavens and the legions of hell. Later he also imbued it with the power to weaken the Nephalem.

It is often speculated that Soulstones are fragments of the Worldstone, but this is yet to be proven. Tyrael originally gave the soulstones to the Horadrim in order to imprison the Prime Evils, but there is no official information concerning their origin.

Destruction of the Worldstone

Since Baal corrupted the Worldstone, Tyrael decided that it must be destroyed. The full effects of destroying the Worldstone is yet to be revealed. Many speculate that Tyrael has somehow been corrupted by Baal or the corrupted Worldstone. All should be revealed in Diablo III

Even the characters in Diablo II aren't sure of what will happen now that it is destroyed, or even if it had to be destroyed in the first place. The Prophecy stated that the Worldstone would be corrupted, but apparently never actually said anything about having to be destroyed.