Destruction of the Worldstone
Available after the end of the quest: The Broken Blade
Inaccessible at the start of The Doom of Wortham.
Deckard Cain:
Tyrael was forced to destroy the Worldstone when Baal, the Lord of Destruction, corrupted it in his attempt to control all of mankind.
Player:
The spirits speak of the land splitting apart.
DC:
It obliterated Mount Arreat, the mountain that housed it, leaving only a smoking crater in its place. And nothing has been heard from Tyrael since.