There is a lever on the wall to open that door in the catacombs. Try turning up the gamma in the video options.

You can cheat to get out of the dungeon. Do you still have a save where you had both shadow potions?
Drag the stack out of your inventory and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number (like -20) and hit enter (the checkmark will be greyed out). This will put 20 shadow potions in your inventory, and a larger stack on the ground.

Alternately, you can get to the exit the 'easy' way;
Teleporting Anywhere, via hex editor


Do you have a save before entering the castle? You will be without the teleporter stones until you can rescue Zandalor. It might be easier in the long run to re-load a previous save.