HoH:MC mentions that some in the Line of Quendalon (p.96) have the Major Magical Focus: Faerie. The question now is: What is covered by that focus? Is it restricted to one particular aspect (Faerie beings, Faerie as a place, etc.pp.) or does it cover everything in the Faerie Realm (dispelling Faerie effects, finding Faerie vis, etc.pp.)?
"Demons" is a Minor Magical Focus, but only covers demons themselves, not Infernal creatures. While this disctinction between "actual demons" and "tainted creatures" is lacking in the Faerie Realm, does it still stand to reason that a Major magical focus should encompass more?
(I guess this is worth mentioning every time: I'm a SG looking for input, so "discuss with your SG" is not gonna help, sorry. )
Let me try to compile a list of possible applications...
Faeries, which is identical with "Creatues with Faerie Might".
Characters touched by Faerie (Faerie Blood, Warping, etc.).
Faerie effects (which might be further split into Faerie powers, Faerie Magic spells, etc.).
Faerie regios (including Arcadia et al).
... and contrast it with a similar list for the Infernal Realm.
Creatures with Infernal Might.
Characters touched by the Infernal (Taint, Infernal Warping, etc.).
Infernal effects (which might be further split into Demon powers, Maleficia, etc.).
Infernal regios (including Hell if it comes to it).
... unless I'm missing something (feel free to expand the list), the conservative interpretation of the Major Magical Focus: Faerie only adds in one list point when directly compared to the Minor Magical Focus: Demons. I agree that basically covering the entire Faerie Realm in a Major Magical Focus might be overdoing it, but on the other hand the other interpretation seems to be closer to a Minor Focus than to a Major. I just wanna see where people stand on this...
"spirits" is a major magical focus. It lets you deal with beings that are usually incorporeal from any realm. I would suggest that a focus that lets you deal with anything that has a magic might be major; similar for divine and fairy beings. "Auras" is probably a minor magical focus; it is a very small piece of vim. I would say the same thing about vis (as a target; I would not allow focus of any kind in all spells that involve vis in their casting, and enchantments, etc.) I would consider "shape shifters" to be a minor focus.
I'm not usually the more restrictive guy. But that doesn't make me right! You are story guide; close your eyes, and reach out with your feelings… no one else here is going to be right either, and that includes the rules.
I would be strongly tempted to allow pretty much everything Faerie to fall under it, rather than only Faerie creatures. With one caveat: I wouldn't allow the Faerie focus to apply to every faerie magic spell cast by the user, ruling instead that the focus only applies to effects that target faerie magic (produced by the caster or others, e.g. dispelling, detecting etc.), rather than to all faerie magic effects produced by the caster.
I would accept MMF:Faerie as a virtue specific to the lineage, covering anything linked to the Faerie Realm of Power. However! This is effectively a Mystery Virtue, and not something that I'd allow to be available "in general."
Thus, I wouldn't accept an Infernal Focus without serious justification (which would require a powerful lineage of diabolist magi that hasn't yet been rooted out to exist in your saga). A Realm focus is simply too broad. (I'd forbid a Magic or Divine focus, period.)
The only suggestion I would make is that also allowing faerie touched/blooded humans into the target substantially broadens the number of forms that would be effected. If you were to scrap them and say things purely of faerie then it would fir better within the bounds of a major focus.