In the downloadable guide to the game, I read that you can learn the demon's true name from Nickademus, BUT I heard nothing about learning it anywhere else, unless you are a Cleric of Talos. However, I was able to learn it from the water genie, Ondabo.

If you have an intelligence that is high enough (my sorceress has an INT and CHAR of 20), you can name the demon yourself when you are speaking to the Abishii leader. He will tell you the name of the demon and you will mention that you know the name and it means Caged Fury. The water genie can tell you have to get rid of the demon. Do not ask for Nickademos’ help for getting rid of the demon, instead ask him to share his experiences which nets you some free XP.

I already got rid of the demon, actually, and am in Chapter 3 now. I learned the demon's name from Ondabo the water genie instead. No one had an int that high.