[Verjigorm]

Astral Corruption, I think, is probably the single most annoying spell in the game considering it's cheapness (because it uses blood slaves) and power. The only truly effective counter is to kill the guy that cast it.

Now... I'm not terribly experienced, and I haven't tried this, but you can wish other people dead right? As long as it's just one person...

so unless it's an immortal pretender, you could just wish him dead for 100 pearls and cancel a 999 slave spell for a fraction of the cost provided your wish caster is immortal or could kill the resulting horror with his guards...

I think I read somewhere that you could wish specific named units dead like...

"Kill Tomaso Del Monferrada"

Does that work?