So as we all know dispelling becomes a lot easier when it's effect specific - the examples in the corebook being "Hermetic Terram magic" or "Shamanic spirit control magic".
How does this relate to other traditions and - more interestingly - creatures? If you want to end demonic possession for example, is "demon powers" (which would probably not include Maleficia) specific enough, or does it have to be "demonic Mentem effects"?
Also how does cancelling demon powers work to begin with? They don't have a base level, so what's the level of the effect? Might Points spent x 5? Might Score of the demon?
That's a tricky question, we recently had the discussion about hedge magic. Philipus Niger in GotF is designed without obeying what core rules say about this!
But to compare with Hermetic magic then you need separate spells for each of the Forms.
So unless dispelling powers of demons (or faeries for that matter) I'd say you need one for each Infernal [Form equivalent] Power, so 10 in all. Just like Hermetic Magic.
And for Gruagachan you'd need one for each hedge art: Curse, Blessing, Shape, Vision...
This is one of those things that will probably vary by saga. If your saga sees a lot of (insert type of foreign magic) as antagonists to the covenant, then the categories will be narrower. If they almost never appear, then the categories should probably be wider.
So if your saga has a lot of demons, then maybe the SG will say that the kind of dispelling will be equivalent to Hermetic Forms (so "Demonic Corpus Effects"), while if they appear less frequently it could be the equivalent of Techniques (so "Demonic Control Effects").
As for the level of the powers, I haven't looked at RoP:I and RoP:F for quite some time. But RoP:M uses an effect level as a base for the powers. As a fallback, you could always decide whether the power is "core" to the creatures (use full creature's Might) or more peripheral (us half the creature's Might). Using the power's Might cost is quirky, since powers central to the creature's concept usually have a reduced cost, so you would need to adjust.
Opposed to faeries and magic beings, demons don't.
But I discovered one example in one of the Apotropaic Spells - Early Punishment for the Sinful Witch (which is a weird spell in itself, given that it bears no relation to the MuVi guidelines)* - which states: "If the Infernal Power does not have a level, then its effective level is [...] (5 x Might Points expended) instead. Infernal powers with a constant effect that require no Might expenditure cannot be affected by this spell."
[size=85]*Shouldn't the spell affect powers with a level of up to half the (level - 2 magnitudes) of this spell, as per the third Muto Vim guideline (ArM5, p.159)?[/size]
In the meantime I found the answer to creature dispelling myself: The base level for dispelling a creature's power is its Might Score (ArM5, p.191).
The other question remains strong: How would you rule creature powers in regard to "specific effects"? I'd like to hear some more opinions on it. The examples in the core rulebook state "Hermetic Terram magic" and "Shamanic spirit control magic" as examples. In regards to hedge and rival traditions I'm inclined to base the specificness of their effects on their forms (Gruagachan Blessing magic or Solomonic Physic magic, for example). But given that creature powers don't have their own "techniques", defaulting to Hermetic Arts is probably the closest thing, yet it seems overly specific. On the other hand, a "dispel demon power" spell using the specific guideline might turn out quite unbalanced. (Using the broader guideline would result in a spell dispelling any infernal effect as long as it's half the level of the spell.) How do others think about this?
Submitted it back then, and IIRC the basic answer was that the spell as written was beyond repair and somehow slipped through playtesting, so it's just best not to try to mess with it.
Regarding creature powers, your dispelling spells do not need to be exclusive. Creature powers do list an Hermetic Form for the determination of Magic Resistance, so you could have a 'dispel magic creature powers resisted by Ignem' spell, that would seem specific enough. On the other hand, going by creature would also work, as in 'dispel dragons' powers' (and only dragons). As for demons, the term is sometimes used for any infernal creature, or any intelligent infernal creature, and sometimes it seems to have a more specific meaning, but you are right it's just too broad to be used for a specific guideline. Better stick to the equivalent Hermetic Form then for their powers (and keep in mind that if they actually use some form of infernal magic, that is again a different category from their innate powers fueled by Might.
I agree, I would be quite happy with (to use your examples) "dispell dragon powers" or "dispell [magic] creature Ignem analogue powers" as being non-exclusive examples of a "specific type of magic".
Demons are grouped according to Order, so I think that I would allow both "dispell [infernal] creature Ignem analogue powers" and "dispell Order of Tempters" powers, as examples of "specific types of [infernal] creature magic".
The latter might actually not be such a bad idea... even though I'm not certain if the actual workings of the powers of the orders would be different enough to make in-game sense for this specification, I still like it more than "Ignem powers" for example.
Well, for creatures' powers, having a high realm lore would seem to be right (and those abilities had better be good for something...)
Remember that having a high Infernal Lore is suspect in the Order, since the study of demons is discouraged, other than studying the weakness of a specific demon that is already causing trouble and needs to be put down.