When I cast Enervation, an 18 appears over the target, and the combat log says that 18 points of negative energy damage was dealt. This doesn't make sense. Not only is it always 18, but but it shouldn't be doing damage. The only thing I can think of that might be causing this is KaedPRC. Nothing else should be altering any of the game data, but looking at what it changes (http://nwn2customcontent.wikidot.com/) makes it seem as if it shouldn't be causing the issue.

Is this normal? Is it actually working?

I'm guessing that the damage is related to the negative levels bestowed by the spell; specifically, the game reconciling the target's current HP with the reduced Max HP associated with the lowered effective level.

(Keep in mind that in 3E in particular the ruleset explicitly supports temporary HP above a being's normal maximum, therefore, an automatic reduction of current HP when max HP is lowered would be a bug, and therefore, presumably, the spell script needs to manually remove the appropriate amount of current HP)