"Are you strong enough to face your own masters?"
Your former masters (see the story at Raven's page), Zedek, Marshal of the Legion; Traductus, Grand Patriarch of the Church; and Menelkir, Arch-Mage of the Arcanum, are waiting for you in their tombs. (Korax gave them the gift of Unlife. My guess is that since "undead" means the living dead, Unlife means the "dead living"... which is probably why these three masters are in their tombs.) Each of them represents one of the three character classes in the game. Zedek represents the Fighter; Traductus, the Cleric; and Menelkir, the Mage. You must enter their tombs and defeat each of them. Then only you can reach your goal: to enter Korax's stronghold, and destroy Korax himself. However, finding your three former masters in their tombs isn't as easy as you think. Each of the tombs has its own puzzles, traps, and monsters which you have to deal with before you actually fight the master.
Before you start solving the puzzles in the three tombs, however, you might want to visit the Vivarium. Note that it's accessible only before you enter any of the tombs. After you enter any one of the tombs, the portal to the Vivarium will be sealed off forever. There's a fair amount of goodies in there (and lots of monsters), so you might want to go there to 'stock up' on useful items before you fight the three masters and Korax himself.
Now go to the northwest of the Necropolis, where there's a door with a panel at the side. There are three outlines on this panel, each matching one of the Glaive Seal, the Sigil of the Magus, and the Holy Relic. Fit these on the panel, and the door opens. There's a portal behind the door. Collect the mana here (there should be plenty -- and you need them!), gather all your resources together, and prepare for the final battle. Enter the portal to the Dark Crucible, the stronghold of Korax: your final objective.