The entrance to the dungeon you are at is dungeon key 6. I had the same problem. If you back track towards dungeon 4 you will see areas that are still blacked out, detour through these and you will find dungeon 5.

I never did find the dungeon 6 key. Finished the game though.


God grant me the serenity to accept the things I cannot change, courage to change the things I can and the wisdom to hide the bodies of those I had to kill, because they pissed me off.