You are missing Aribeth in the dungeon apparently

(sorry - couldn't help it)
I don't know what is wrong if Aribeth isn't there, did you kill her or something ealier when confronting her - or did you convice her to give herself up to Lord Nasher? Is your game patched fully - otherwise it could be you have a glitch.
I don't recall her playing a major part in the game at this point, so you don't really loose much by not talking to her in the prison.
(By the way - I've added as spoiler tag to your thread-title)