Greetings,

 

during my Adventures I came across the above mentioned Dungeon. I took care of the Necromancer down there, but eventually I got stuck at this door:

 

 

 

Even after recieving a key for realeasing a young prisoner, and taking down Raedric, I was unable to open this door. Did I miss the key, or am I not supposed to go in there in the first place?

You didn't miss anything, that door cannot be opened at that time.

 

 

It actually only opens with a lever from the other side, I don't think there's an actual key at all.