I'm having trouble understanding where Exorcising the Filthy Spirit comes from.
Under the possession power, it says "Vim spells may be used on the energumen to target the demon rather than the host; these must overcome the demon's Infernal Might [...], but drain the Possession Might Pool, not the Infernal Might."
In the Black Magic guidelines, there's a guideline for removing the Might Pool of a target, but we also have Exorcising the Filthy Spirit which doesn't seem to make sense. It seems to imply that the Possession Might Pool behaves very differently from normal Might Pools, even from a magical point of view.
Can anyone explain why you couldn't just use a might pool reducing spell on the energumen which the targets the demon, or if it doesn't work that way, design a might pool reducing spell specifically for targeting demons that are possessing someone that removes base level + 2 magnitudes of Possession Might Points?
My own understanding is that "Exorcising the Filthy Spirit" is an example of a spell that uses the guideline you've referenced.
If I understand your question correctly, however, the reason for EtFS is that it specifically targets the Might Points that the demon invested into empowering its Possession. This could, potentially, be much less than the demon's total Might Pool and so the spell is more useful at lower levels where the caster can achieve a higher Penetration.
But if it uses the guideline, why does it remove Possession Might Points equal to the spell level minus the demon's Might, rather than simply removing points equal to the spell?
Also, the existence of EtFS implies that any anti possession spell needs to be specifically designed to be just that, rather than letting you use a general spell like Lash of the Chastised Servant on the energumen to target the demon. Would you say this is a correct assessment?