Bleeding the Demon
Re(Pe)Vi, Voice, Momentary, Individual
Weakens and possibly destroys a demon. If the spell penetrates the demon's Magic Resistance, the demon loses Might equal to the spell's level - 10. For every 5 levels of Might lost in this way 1 pawn of vis is extracted from the target and transferred to a receptacle on your person.
(Base: 10, +2 voice)
Not sure this is possible or balanced, but it is meant to be a blend of Demon’s Eternal Oblivion (PeVi) and Gather the Essence of the Beast (ReVi).
The idea is that the PeVi component separates the demon from his might and the ReVi transfers it away in the form of vis. The problem with using DEO by itself is it leaves no vis behind as a prize for the winner (IMS). Leaving aside the issue of infernally tainted vis for the moment (something that won't be an issue for the magic realm version of this spell), is this possible and is the spell level accurate?
Base level 10: Move raw vis from one physical object to another without needing a laboratory.
General: Reduce a target's Might by the level of the spell + 10 as long as the spell penetrates the creature's resistance.
Demon’s Eternal Oblivion
Voice, Momentary, Individual
Weakens and possibly destroys a demon. If the spell penetrates the demon's Magic Resistance, the demon loses Might equal to the spell's level.
Gather the Essence of the Beast (boosted to voice range)
Voice, Momentary, Individual
Concentrates the raw vis in a corpse into one part of that corpse which can then be removed.
(Base: 10, +2 voice)
For me, this is the critical point. In your saga, is the fact that DEO destroys the vis meant to "balance" the fact that it is often easier for a magus to DEO a demon than to "kill it" with mechanical damage. Is your troupe happy with this spell shifting the "balance" in this way? Which is something that really only your troupe can answer.
There seems to be something conceptually odd about a spell that both destroys and moves "magic power"/vis simultaneously. If the the vis is destroyed, how can it be moved?
Actually, this is a really good reason for spell like this to appear to work, but only with some demons (and not other realms). That is, a demon might use illusions to make it look like the spell has worked as a way to deliver some infernally tainted vis to the magus, which the magus can then damn himself with.
I'll tell you that i've thought this before, and to a the four realms version: mariojpcsimon.blogspot.com/2010/ ... or_25.html
Extract the Enchanted Anima (against Magical Beings)
Steal the Ambrosia (against Faeries)
Trap the Angel's feathers (against Divine Beings)
Milk the treacherous Ichor (against Infernal beings)
All are similar the being loses a stress die of Might Points and the magnitud pass on the form of Vis to the caster (the Infernal beings lose 5 more). All are Re(Pe)Vi 25.
And i almost forget to say it... al have very bad consequences if are botched, and if are succesfull too.
First of all, I agree with Richard Love about the fact that an effect that transfers vis from a demon to the caster should not include a Perdo requisite. If you mean to steal for yourself the magical power of the demon, you should not start by destroying that very power -- just like a magus who wanted to steal the leather boots of another andgainthemforhimself would use ReAn, not Re(Pe)An (despite the fact that you can kill an animal with PeAn, and then make boots out of the carcass with ReAn).
However, stealing some things is much harder than stealing a pair of boots. In particular, it seems very hard for standard Hermetic magic to steal energy (youth, vis, fatigue levels) from a living target. I do not think that standard hermetic magic can achieve the effect proposed by the original poster, just like standard hermetic does not allow a magus to steal an opponent's fatigue levels, health levels, or years of life. Sure, using Perdo a magus can destroy those things in an opponent, causing fatigue, inflicting wounds, or causing the target to age -- but he cannot make them his own, becoming less fatigued, younger, or healed in the process.
Note that this is specifically a limit of standard Hermetic magic. There are many such examples of "stealing" in Infernalism, including transfering fatigue, decrepitude and wounds from one target to another. Also, there are some Mysteries that allow a magus to achieve something similar, albeit at much greater cost: for example Hermetic Empowerment (TMRE) allows a magus to enchant items draining bound spirits to create Ritual effects, and Bind magical creatures (HoH:MC) allows a magus to drain the lifeforce of a living magical creature to provide the vis for an enchanted item.
I would say "no", on several grounds - which were largely raised above.
Primarily - I don't want such a spell to work. Not being able to drain the demon when you destroy it is much more interesting, game-wise, I think. But even if this wasn't the case - the whole spell just shouldn't work.
You're attempting to combine two different effects, so this should garner at least one extra magnitude - probably at least two in this case, I'd say. But the problem is that the secondary effect - separating the Might - isn't something Hermetic magic can do. You'll probably need a Mystery, like the Binding mentioned above, and even then precedent says it should take a Season. Hermetic magi just aren't that good at moving raw vis around.
If you do decide to allow this as a spell, I can understand using Perdo to "disengage" the demon from the raw vis, but... not at that guideline. It should be harder. I'd raise 4 magnitudes for complexity, too, or else require a delicate Finesse roll (perhaps also some supernatural vision). Destroying the demon's might is easy; separating its essence from the raw power its spirit accrued is a far more delicate job.
At very least, if you're going to use this there should be added magnitudes for the requisite. You can't just use the same base 10 with +2 Voice and get lots of other stuff just by tossing in a Perdo requisite.
I don't think you should use the general PeVi guideline 'destroy Might' to do this, since it is supposed to, well, destroy the Might and therefore the associated vis. But if you used a PeVi guideline based on the PeCo/PeAn guidelines to cause a Wound, then you could cut off a 'piece' of the demon (or the demon's essence) and siphon the vis from it :
PeVi 5 : cause a Light Wound + ReVi 10 : siphon vis -> Re(Pe)Vi Base 10, +2 Voice, +1 Perdo Requisite, total level 25: cause a Light Wound to a demon, cutting off part of its essence, and siphon one pawn of vis from the cut-off part.
RoP:Magic has some interesting rules in the 'Temporary Might' Inferiority (p.42) for Magic creatures regarding how much vis you can gain by cutting off part (by causing wounds), the first light wound nets you a pawn but then you need heavier and heavier wounds... note that this implies that only creatures with this flaw can be injured to produce vis in this manner, but if you really want to siphon infernal vis, presumably to use afterwards, I know quite a few GM who will gleefully help you while smiling inwardly at all the problems they'll soon drop on your head.
My approach was near to the Rego Corpus from A&A to magical Chirurgy and take things on a distance, and Perdo applied to bodys and wounds. To my spells would be worrk a Muto or Form requisite to the physical matter of the beings affected.
Any Player of Ars should think as a StoryGuide whenever designing a new spell, and certainly when trying to create something radical like this. Does this effect fit within the AM world as described? Is the effect so obvious that it should have been invented long ago, and if so why hasn't it? Would it require a breakthrough (and, again, if hugely advantageous why hasn't such been achieved before?) Will it change the way the game or game-world works? (There may be easy answers to these questions for your Saga/Troupe, but they need to be answered so the Troupe is comfortable with them.)
This type of effect specifically can lead to an easy solution around the whole question of vis - vis farms/ranches, etc. Unless a Troupe (SG included) want to brush aside the whole question of "vis as a limiter" and revolutionize the Hermetic vis economy, not a good plan imo.
Variations on this suggestion have been proposed before, and raise several questions that need to be answered first. Do all creatures w/ Magical Might (from whatever Realm) have vis within them? Is vis harvestable from all creatures w/ MM? What happens to a creature with Magical Might (from any Realm) when they lose all their vis? This last is, perhaps, the most crucial. (And, no, you won't find these in any book afaik, nor am I going to suggest a "correct" answer.)
However, to play devil's advocate, if vis can be "moved" from one part of an animal to another using ReVi, or similarly from one object to another for storage, then there is a logic that suggests it can be "teleported" further than the bounds of that creature (if at a much higher magnitude of base spell effect) - so it's not counter to the RAW (just to the sensibilities of many SG's.)
I agree, there doesn't seem to be anything conceptually wrong with using ReVi (at some magnitude) to move vis around. In fact, the standard ReVi 10 guideline from the core rules "Move raw vis from one physical object to another" does exactly this.
Just to emphasise what you say, I see the main questions are:
a) whether vis embodied in a living magical creature is raw vis that can be targeted. Probably it is, as it is certainly raw vis when it is inside a dead magical creature --- but perhaps it only becomes "raw" vis when the creature dies.
b) whether or not all magical creatures have vis in them or not is a moot point. Most do (or at least most published ones do). If some don't then obviously they won't be a viable target for spells that target the vis in them. Just like a person without feet is not a valid target for spells that target feet.
c) I agree that an important question, which seems to be unanswered in the rules, is what happens to a living magical creature when some (or all) of the vis embodied in it is removed.
There are some answers to this one, though vague, in RoP:M. Some will die while others will be fine but non-magical. Also, there is another book with a comment about spirits of some sort losing some but not all of their Might. I haven't been able to find the latter again.