Quest for Glory IV: Shadows of Darkness/Endgame

After completing the major events: obtaining the rituals, and rescuing Tanya, you're ready for the final stage of the game. After rescuing Tanya you'll eventually get a letter from Katrina asking to meet by the castle gates at night.

You'll run into Ad Avis when you go there, and he'll send his necrotaurs after you. You can escape back to town if you have levitate and you're really smart, but you'll need to get captured to move on with the game anyway.