Ack, my bad. It's not there, so I'll just dock it; sacrifices already get death and all the other aero-related spells, so it's not like they need a whole lot more.
As for Imperil, it flat-out replaces any immunity or resistance with vulnerability. It's not a permanent thing though, so it's not quite so huge of a problem. I mean, yeah, it would be a bit fairer and more accurate to make it downgrade immunity to resistance to non-resistance to weakness, but it's simpler and easier on the GM's bookkeeping to just slap "VULNERABLE" on an enemy instead of keeping track of everything.
As for Imperil, it flat-out replaces any immunity or resistance with vulnerability. It's not a permanent thing though, so it's not quite so huge of a problem. I mean, yeah, it would be a bit fairer and more accurate to make it downgrade immunity to resistance to non-resistance to weakness, but it's simpler and easier on the GM's bookkeeping to just slap "VULNERABLE" on an enemy instead of keeping track of everything.