Ah! The Phase Spider teleport...good suggestion. Deathdrinkers (the type of demon Prisoner-Slayer is) are supposed to teleport and I couldn't find a good way to make that work. The Phase Spider teleport is a good enough script.
Here are the changes I've made (no idea yet the effect on balance but CR is more or less the same, just 1 less):
-Turned the 10 cleric levels I had given him into 10 barbarian levels to make him more of a tank. I realized that those 10 cleric levels were sort of a waste since the spells were hardly good enough for the level of people fighting him.
-Gave him a Pulse, Negative since the actual creature is supposed to have an Aura of Negative energy, which is not in NwN.
-Gave him the Phase Spider type of teleport.
Aside from that I didn't retouch him much...he should become more of a tank now, a pure tank. For now, I won't retouch the area itself or the spawns surrounding him; I'll wait to see the impact of the changes.