A slight inconsistency at very low levels, yes. Technically, the level 5 spell should lower Might by 13 ("spell" base level, plus 10). A level 10 spell (4+2 voice) should lower Might by 14, a level 15 spell should lower Might by 15.
Although inconsistent with the actual rules, I would actually prefer to read the guidelines as "spell level+2 magnitudes", so a level 10 spell (4+2 voice) would lower the target's might by 10 points and so on.
I bet this is covered in Realms of Power: The Infernal , which I don't have . I would bet the farm (but not my soul!) that it definatly works on a demon in disguise, though a demon might be able to cover up that it was hurt until it was banished.
As for a possession, this might require a special PeVi spell with a mentem requisite.
Yes - provided the magus has somehow first 'sensed' the demon or holds an arcane connection to it (a Lesser Limit of Hermetic magic).
This is evident both from the spell description on ArM5 p.160, which makes no reservations about which demons in which form are subject to DEO, and from the summary of the working of Hermetic magic on demons from RoP:tI p.33.
So only a demon leaving no arcane connections around and hiding carefully and completely is immune to DEO.
Ok, now you have ME curious. Does that mean that you can't Phylum of Fire on a demon in an altered shape (or at least damage them)? Or is it you can only damage their Magic Might if you know their a Demon and have an Arcane Connection?
Is this a property of Demons or Vim?
As far as possession, I'm not sure that the demon's Magic Might is present on the person being possessed to be damaged. It wouldn't suprise me that some Demons possess by spell-like magic and some actually take up residence in the possessed brain. Is this covered in Realms of Power: The Infernal?
You could. It is when demons aren't in physical form that you will have a hard time affecting them: you cannot affect something that you cannot sense, and Hermetic magic cannot detect demons. But if you see a puppy in front of you, feel free to cast PoF or DEO at it as much as you like, whether or not you know they are demons or not.
A combination of infernal deception and the need to sense your target.
Yes. Possession is covered. All demons can do it by investing some of their Might in one or several victims. It is nasty. DEO needs to explicitely target the possessing demon, penetrate the combined MR of the demon and possession victim and will only destroy whatever Might the demon invested in the victim rather than destroy the demon himself.
It normally doesn't make sense to cast Pilum of Fire on beings without a body, as the spell would not harm them. Demons in spiritual form are no exception here.
You can, however, aim a Pilum of Fire at anything you can sense, also on a being which you suspect to be a demon in altered shape. If you thus fry an innocent fishmonger whom you mistook for a demon, hell exults.
If you have an arcane connection to a demon, you need not sense that demon to target it. Otherwise you need to sense it to make it the target of your spell. Unless you have specific non-hermetic supernatural Virtues, sensing demons is impossible if they are in spiritual form and do not give you a hint of their whereabouts or even existence.
Neither. it is a Lesser Limit of Hermetic magic, exacerbated by demonic deception.
That's treated in RoP:tI p.32. The demon invests Might into the possessed person, and if a magus can sense that possessing Might of the demon or has an arcane connection to it, he can DOE it.
If you have an arcane connection to the demon, 'knowing it is there' is not even needed. If not, 'knowing it is there' - in the possessed person - is not sufficient. After all, you want to target the demon, not the possessed person, with DEO. So you must sense the demon, not the person, to cast the spell.
AFAICS, if the demon controls the possessed person and makes her do something or uses specific powers of his, a magus senses that demon at that moment and can target it. But if a demon hides within its host, leaving her to act on her own, the demon cannot be sensed at that time, and targeting the host with DEO does not affect the possessing demon.
Targeting a suspected host with Pilum of Fire is of course always possible, and always fries the host, never the demon. No matter whether you sensed the possessing demon or not when you cast that spell.
I had considerd creating a target structure version of DoE for a character with a spell level of five or less and casting it (with the resulting high penetration) fairly ferrequently withing the covenant or any buildings where he was spending any considerable amount of time.
The campain ended before I had a chance to do this and I was weighing the possibility that my character would be attracting unwanted attention against the possible benefit.
Consider the following:
A demon has gained a toehold into a person...
The breach isn't large enough for full out possesion, but it periodically tries to exert control or glimpses of the demons emotions are shown/felt...
Casting DoE the demon can just "step out" of the body for a moment or two evading the spell and the source of entry is still there. How would you get rid of it besides a divine exorcism?
rego vim to bring the demon forth then squish it
trap the demon within a ward
fool the demon with trickery
a rego mentem spell targeting the demon set up within a watching ward spell
enchant a device that casts DeO on the wearer set up with an environmental trigger to activate every time the wearer breathes and unlimited uses per day
get a skilled practitioner of Ars Goeta or a shaman/spirit master to help you out
Some other divine power asistance outside of exorcism