Find Daegarn in the Ring of Judgement located inside the Utgarde Catacombs.
You might have already seen the room where Daegarn is being held while you were gatherin' the tablets. It's through this corridor and then down a set of steps. Daegarn will be in the room at the bottom of the steps - locked up tight!
Get down there and find him. At the very least you might be able to get the cipher from him.