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.
Yep. You found me. Woopedy-doo!
And no, I don't have the cipher. My beard was the first thing those beasts checked! Oluf's the one that's got it now. He's the chieftain of the gladiators.