Arhu the cat gave you the quest to free Zandalor - and told you to visit Rob/Ducal Inn/Verdistis to flag the Ars Magicana sewers for you (flag for treasure room).
I guess, you're in the castle prison, right? Killed the executioner, right? That's not the correct place, no keys.
Or you're in the wrong sewers: Verdistis - 2 entries for the assassin/thieves guild can only be accessed from above because of a lever.
It's more south, east of the hatch leading to the poor quarters/Lain's house in the Ars Magicana sewers..
Kiya
