There are three warriors at the Summit of Mount Arreat that protect the entrance to the Worldstone Keep. Their names are Korlic, Madawc, and Talic. The three warriors were summoned by The Ancients to go to the summit and protect it, honored as they were they departed to Guard the Worldstone for all eternity. When you reach the summit you must face the three warriors in battle to pass on to the Keep.
While the Barbarian Ancients would have delayed Baal, he received a key from Nihlathak that allowed him to pass.
Diablo 3 is finally coming out. Now we only have to wait for a release date :/