Vaults was only a test. Now for the real thing. When you killed D'Sparil in Heretic, you upset the balance of power among the serpent riders. Korax gained too much power indeed. He has frozen D'Sparil's world. The Caldera is locked up tight, the power grid crystals are frozen and unreachable. Your mission: should you decide to accept it, is to restore D'Sparil's heart to its proper place, reawaken the mountain, set the machinery in working order by retrieving the banished parts from other worlds and replacing the power crystals into the power grid. Only then can you face and defeat Korax for real.