Available after the end of the quest: The Broken Blade
Inaccessible at the start of The Doom of Wortham.
*NOTE: You can use the quest choose method to start at the beginning of Doom of Wortham, just don't talk to The Stranger, since when the quest officially starts, Deckard can't be talked to anymore.
The Worldstone was an ancient artifact of immense power. It was used not only to create our world, but also to control mankind.
The original humans were known as the nephalem, and their power was such that it was feared they would tip the balance in the Eternal Conflict between the High Heavens and the Burning Hells.
So the Worldstone was tuned to lessen our powers, turning mankind into what we are today. But now that it has been destroyed, it is conceivable that the powers of the nephalem will reappear.