This is far away from the original spell it seemed to be based upon, which is Foresight. I'm pretty sure the script for Premonition could be altered to function similarly:
Basically, in code, instead of the stoneskin effect, this spell would apply immunity to sneak attack, plus the applicable AC and Reflex save bonuses. In a MUD codebase I developed, I further added the effect of a perpetual Find Traps, though, unsure if that could be done in NWN without further gumming up the heartbeat script.d20SRD wrote: This spell grants you a powerful sixth sense in relation to yourself or another. Once foresight is cast, you receive instantaneous warnings of impending danger or harm. You are never surprised or flat-footed. In addition, the spell gives you a general idea of what action you might take to best protect yourself and gives you a +2 insight bonus to AC and Reflex saves. This insight bonus is lost whenever you would lose a Dexterity bonus to AC.
In any case, fortification against sneak attack in a spell would be IMO way more flavorful, and even useful, than "yet another damage reduction effect."