Very cool. I'd make the save a one time thing, on the first hit, and if the target fails, it's 'real' for them for the remainder of that encounter.
There are definitely not enough Shadow-themed items, and with the existence of Nidal in Golarion, I think there's a lot of potential to play around with items that involve shadow conjuration / evocation, etc.
Yeah, I've been thinking and probably something more like that for the save would work much better.