I've always heard that you can't kill Daedras, only banish them back to Oblivion. I assume it means that their spirit leaves Nirn and goes back to Oblivion, and they leave a mortal remains here on Nirn, So what happens when you trap their soul in a gem? The corporeal body that was on Nirn is dead, the soul is trapped, so doesn't that mean they can't come back ?

Or does the banishment works only for Daedra Princes ?