Destruction of the Worldstone
Available after the end of the quest: The Broken Blade
Inaccessible at the start of The Doom of Wortham.
Tyrael was forced to destroy the Worldstone when Baal, the Lord of Destruction, corrupted it in his attempt to control all of mankind.
The spirits speak of the land splitting apart.
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.