I have a magus concept in front of me that I would appreciate advice on whether to allow or not. I am wondering if there are consequences that I am not yet seeing.
The proposed Magus is a necromancer, whose Sigil is "enhancing the life and vitality of the target". Explained as being something like Kubera's ability in Lord of Light - a chair would exhibit more chairness. A person would be perceived as closer to a paragon of personhood..
I thought it was a setup to justify a positive side effect, but instead it is supposed to justify the Major Flaw - Supernatural Nuisance (Vitality hungry faeries).
I will start by answering the last question first.
It could be a problem because you seem to be using the word "vitality" to mean two (or three) different things.
When you wrote "enhancing the life and vitality of the target" I understood vitality to mean "life-likeness" which poses a problem of "how will the sigil manifest when the magus casts a spell on something that is not naturally alive?". You then specified vitality to mean something like "conformation to the platonic ideal of whatever the target of the spell is an example". Then at last you specify vitality to mean "the thing faeries consume to survive and grow".
I personally dont thing that the "vitality" consumed by faeries bears any relation to a person or objects level of conformation to the platonic ideal of their thing. So already at that point you have a problem.
For me the vitality that faeries consume is a realness that comes from the fact that humans have Souls (as in the Divine soul that can pass to heaven) because this Soul is also the origin of things like creativity and true feeling (and True Feeling), and faeries being essentially fake lack those things but need them to survive. In a sense a faerie needs a role to fill but is completely unable to come up with a role on its own and completely unable to change the role without outside help. It needs humans to come up with a story for it to act out and once it has taken a role in that story it is unable to come up with variations of the role until and unless a human has come up with that variation first. In a sense it is a robot reading out lines from the script with no thought given to how these lines fit into the larger play.
I will not 100% guarantee you that what I just stated about faeries is the party line on faeries but it is at least reasonably close.
This interpretation of the "vitality" consumed by faeries is fundamentally incompatible with the vitality you propose for a sigil. Because whether or not something conforms closely to its platonic ideal does not in any way transfer creativity or Soul to it.
In short your problem is that you use the word vitality to mean different things and just because these different things are referred to by the same word they dont actually have anything in common.
On a lesser note the term "vitality hungry faeries" is an oxymoron kind of like "magical wizards" all faeries are vitality hungry, that is part of the definition of faeries in ArM5. Which means that the flaw in question is really "Proposed Major Flaw - Supernatural nuisance(Faeries)". Maybe it does not matter or maybe the player in question is trying to use linguistic tricks to make the flaw seem bigger than it is. Personally I would have no problem allowing a flaw like that but that is something you would have to consider.
It seems from your OP that you are going to be the storyguide so you will have to weigh this for yourself. Especially the questions of "Am I going to actually introduce faeries to bother the player to the degree that I (you) personally feel justifies a major flaw?" and "Do I care?". You should also talk to the player about what they want out of this flaw, because if you actually play it out it is going to take a lot of effort of the storyguide to come up with good faeries and if the player takes the flaw expecting lots of good faeries and you dont provide them then that sucks. But on the other hand it is also possible that the player took the flaw in an effort to "get off the hook easy" and acquire some virtue points with which to buy cool virtues. There is no way for me to know which but it matters and you should try to find out.
I should say that I dont view it as a bad thing to try to "get off the hook" in regards to flaws, Certain Personality and Story flaws seem to pretty much be in the game for the explicit purpose.
If one's magic creates the Vitality all faeries crave, this might make faeries crowd one, but it will make them bargain with and protect one as well.
How much worth this is as a bargaining chip depends on the saga - but it may be enormous and determining one's position in Mythic Europe.
It is partly the lack of mechanics for Vitality (as used by Faeries) which had me uncertain. I do like your description.
OTOH a human mage has a Soul, has to invent/create their own spells, and the Gift is occasionally described as a Divine Charism. So kind of a match to your description.
If I recall the somewhat hesitant explanation, I think they were suggesting that the spell Sigil is supposed to make things more, if not bringing about a closer "conformation to the platonic ideal of whatever the target of the spell is" (which was approximately my first thought on hearing the description), then more like that the Target becomes more to the foreground and momentarily like an active story element for any nearby faerie who uses that sort of prop in their story. I am having difficulty repeating the description accurately.
The thing is that in faerie it is defined that things created by magic (unless possibly if vis is involved) lack vitality (faerie definition), so bread baked by craft magic lacks the faerie-craved vitality of bread baked by a baker using their own hands and time. To have magic which ignores this limitation is, in my mind, more than a sigil alone.
I agree that a magus inventing spells create vitality that a faerie can consume in the same way that a baker who bakes bread creates vitality. I don't agree that this implies that spells can create vitality (in the faerie sense) as a byproduct merely of being cast.
Though spells obviously can create vitality as byproduct of being cast if the casting of the spell causes some outcome where humans act in a way that involves emotions and creativity and other storyworthy actions.
For example casting a "pilum of fire" on someone and grievously injuring them is likely to cause the injured person to experience a lot of emotions (anguish at the pain, various antagonistic emotions towards the caster) and these emotions create vitality, likewise if the injured person acts in a way that makes for a good story or other people act upon the injured person in a way that creates a good story then that creates vitality too. However I dont think that the sigil-ified burnt residue left by the spell should contain vitality under normal circumstances and in particular, I personally think that something more than a sigil is necessary in order to break out of the "normal circumstances" limit that I just made up.
I do think that you could have a sigil that makes things look more like an ideal version of what they are. I would call this sigil something like "orderliness" or "idealizing" rather than "vitality".
You are of course free to disagree with my stance on the matter as well as my choice of words.
Having had the night to think things over I have some points I would like to raise with you.
I think a fair analogy to a sigil that creates "vitality" is a sigil with an effect that prevents Magic aligned creatures from suffering "acclimation". Having reached that conclusion I would not allow that as a sigil. However it could fit under the description of the "Side effect" virtue IMO, I would be dubious about it in my own saga, but nonetheless I could see why others would want to allow that.
Second PC's are by the nature of the game vitality factories since PC's tend to engage in stories with lots of drama and tension, so in a way your magus probably has the ability to generate lots of vitality for free, just by the nature of the game. So it is only fair to take a flaw to represent getting haunted by faeries who have realized this.
They must be, otherwise there wouldn't be virtues and flaws relating to the Gift penalty in HoH: Mystery cults section for Merinita. These virtues & flaws imply fae, infernal, and magic spirits are all affected by The Gift unless they specifically have "Unaffected by the Gift".
I have taken the advice and nixed the initial sigil idea.
The casting sigil is now "infusion of rich blood", and the character now has the virtue Side Effect - +1 Stamina, though they still wanted the flaw Supernatural Nuisance, so currently defining it as a subset of blood thirsty supernatural creatures attracted by the supernatural scent of blood. Maybe ghosts seeking (temporary) corporeal form.
The sigil has healthy, rich blood appear in the target - people get flushed/blush, stones exude blood from cracks, etc