The mystical robes are found during the Ghosts Ahoy quest. After showing Necrovarus the Petition form with 10 signatures, he burns it to ashes and then drops the key, which you use to enter the room upstairs. Once in the room, open and search the coffin to find the robes. Upon giving the Old Crone the robes, any duplicates will be removed from the inventory or bank; however, completing the quest does not eliminate any duplicates.
For an exhaustive list of all known sources for this item, see here