It used to strip PfA before the changes and incorporation of Shell. I've also tested it and if you stay in the fog the save only triggers once when it's first cast and not repeatedly each round. There's no reason to ever cast it except as a RP spell during a ritual, as you need to beat a will save for it to do anything and the effect is just -10 to future will saves, whereas if you cast another will based spell, you could do something such as entirely disable an enemy.
I would suggest:
That it strip Shell
That its saves trigger every round like other fogs
or for something more radical:
That it deliver -5 will saves without a save