Quest item: Warden's Notes. Obtained from the Torture Chamber in the Underworld.

The item remains in the inventory even though the quest has been completed. It cannot be dropped, destroyed or put in the shared storage. That's one precious storage slot eaten up by a worthless item!

I understand the Torture Chamber has been massively revamped, and if my memory serves me correctly, in the previous version of the game there was no problem with such a bug. I believe you had to read the warden's notes on a bookshelf.