Corpses?

[Psitticine]

There's also a nifty spell called Feast of Ravens. It summons an unkindness of ravens to go and consume all the unburied corpses in a provinces, and then return to be slaughtered for the concentrated Death energy they'll then be filled with.

That not only denies necromancers from being able to raise many kinds of undead in that province, but it also gives you a good fistful of Death gems. Heh-heh!