Still enjoying, found another one. This time, I found the Lich's source of power but I never negotiated with him, I killed him (I advanced a lot in the story before doing that, it is almost among the last sidequest I am doing) and took the key, yet the game is writing as if the lich sent me there.
Also, I saw the gallery and I think it would be nice to know how to unlock some of the requirements to unlock them