He is not immune to death magic, he just has high saves, and therefor rarely fails them.
I killed him by using the Black Blade of Disaster where he failed a fortitude on hit, which destroyed him (remember a roll of 1 equals fail, no matter how much bonus there is).
Strangely enough -then the spell description in NwN dosen't speak of this feature, but BG2's desciption of the spell does
