Would it be possible to invent a spell that makes no sense, specifically with the intention of altering it with Muto Vim magic at the point of casting to a spell that does make sense?
For example, a mage wants to cast a Group target version of Demon's Eternal Oblivion but their Perdo isn't quite good enough to get the penetration required. However, they could cast a "Part" target version (lvl 15 rather than lvl 20, for example) and then use a Muto Vim spell to change it up to "Group". The thing is, a DEO which targets part of a Demon's might doesn't exactly make sense.
I don't like it. I see what you're doing, but I'd want a spell to make sense on it's own. Only use T:Part when it makes sense. In a Troupe I would complain abut that spell.
But this is a well known problem, that Group is two magnitudes above Individual, with part in between. And it makes sense that Part is actually harder than Individual. But also Flexible Formulaic Magic is a pain, because none of your T:Ind spells can be bumped up to Group. IMHO this tiny thing makes FFM weak. But I'll stop rambling before I go too much off topic.
I’d support a DEO that uses T:Part, as its not going to be better than the T:Ind version anyway. And the extra mags are hurting the Penetration. You’re affecting part of the demon not the whole, just like Part of a mountain.
The bit that I dislike and don’t follow is how the same caster can penetrate with T:Group. It’s still 5 levels higher which reduces penetration
I can try! To begin with, I can fully accept that I'm getting something wrong somewhere so feel free to point out any errors in what follows.
The situation: our heroine wishes to cleanse an island of demons - we're doing the Rhine Gorge saga and there's an island full of demonic mice. She can easily cast a Individual target DEO but realised that a Group target would be even better. She's adequate at Perdo but really good at Muto. Accepting that a few seasons of study can change the situation, let's say that time is of the essence and she's trying to cleanse the island as quickly as possible.
The calculations thus far:
A DEO that strips 10 Might from a Group is PeVi20 (base, +2 Voice, +2 Group). To get through the 10MR of the demons, she'd need to hit a Casting Total of 30 which is unlikely at this time.
I had the bright idea that she could invent a Part target DEO (PeVi15, base, +2 Voice, +1 Part) that strips 10 Might. She could then cast that alongside a MuVi "Wizard's Scope" spell (like Wizard's Reach but for Target) that boosts the Target of the spell from Part to Group. My understanding of the MuVi change is that the DEO would then mutate to a Target: Group but it would still be cast as a level 15 spell.
She could probably hit a casting total of 25 on this boosted "Part" DEO which, in my understanding, provides the 10 penetration necessary to zap a group of 10 Might demons.
GM/troupe: Can the magus even explain what they're trying to do with the spell? I get that the player can cite a guideline and a Target to show things mathematically. But can the one who's actually inventing the spell figure out what's being done. If not, should the magus even be able to invent it?
Player/Character: I generally figure DEO and similar spells may well get cast in an Infernal Aura. So it's likely to get some extra botch dice. Also, DEO is likely to be used in combat, when there is not a lot of time to spare and it's also riskier (perhaps more botch dice or Concentration rolls). So I don't see that putting a MuVi effect with its extra casting and roll and increased botch dice on top of the DEO effect is worth it at all. 15 experience in Spell Mastery pretty much gets you to a much better spot. Choose Penetration and Imperturbable Casting. The 2 points of Spell Mastery give you +4 to penetration, 2 fewer botch dice, and +2 to your Concentration rolls in case you're wounded or similar while fighting the demon and its minions. You can get most of 15 experience in 1 season from a tractatus, and it's a pretty common spell.
AFAICS you use T: Part for no other reason than to allow a standard MuVi spell to lift it up to T: Group, right?
Look up:
Suggest to your troupe, to invent a T: Pair DEO (PeVi15, base, +2 Voice, +1 Pair) that strips 10 Might of one or two targets. You need your troupe's approval, but it is plausible that you get it. That T: Pair spell makes then sense on its own, and you can use the MuVi spell you wish on it to expand it to T: Group if needed.
The tricky issue remaining is then, whether the Mutoed DEO would penetrate as a level 15 or a level 20 spell. Point your troupe to ArM5 p.159 Sorceror's Fork (with the erratum) as input for their decision.
Creating a spell with Ind plus an extra magnitude for a really big Ind ought to work here. The underlying spell makes sense, and going from Ind+1 to Group also makes a kind of sense.
DEO with T: Group - the objective of MorganN - would allow to simultaneously attack 10 separate demons independent of their size (ArM5 p.156f Vim Spells).
To simultaneously attack 10 targets with ArM5 p.86f Spell Mastery and Multiple Casting requires a DEO Mastery Score of 9, hence 225 xps into that Ability. This is beyond the reach of typical magus PCs, who would rather put such xps into Arts, Magic Theory, Penetration and/or Finesse.
Well, maybe less than 10 attacks at once could do it? I mean, if you wanted to play it safer and could live with slower as well. It would use standard DEO, a spell there should be an easily available lab text for, as well as probably books on mastery.
Compared to inventing a non-standard parameter version, as well as a Muto Vim spell. I know there is some disagreement about whether you cast a Muto Vim metamagic spell on the same round as the target spell. A thread a good deal of time ago David Chart wrote than the intention was for the target spell to be cast and somehow "held" and bot coming into effect until the next round, when the Muto Vim spell is cast and affects it. Although I don't agree, and find nothing in the MuVi guideline box to convince me.
But you need to cast your T:Pair DEO and at the same time cast MuVi, requiring a concentration roll. So that's 3 rolls each round with a risk of Botch, with added Botch dice due to infernal aura (which I assume there is).
It's risky, but I like it. We tend to not do any special parameters in my Troupe - but I like that Target: Pair (equivalent to Part) a lot better than the original idea.
Go for it!
But maybe consider Imperturbable Casting? Which brings me to the situation think I asked, back when David Chart gave the answer above: Is it the Imperturbable Mastery of the DEO or MuVi which affects the concentration roll?
Yes, changing spells according to the errataed ArM5 p.159 Muto Vim Guidelines is indeed tricky and botch prone for both players and characters.
IIRC, an old post by David Chart on the Berklist was once quoted without reference on this forum, which said that a caster had to first cast the MuVi spell and only in the following round the spell she wished to modify with it. 8)
MorganN did not ask our advice on using MuVi, though.
Tangent to explain the in-character situation a bit more ...
There are a few factors at play here. One of them is this guidance from GotF, regarding the swarms of demonic mice: "Each swarm of the demonic mice that inhabit the Mäuseturm should be treated as a single creature, and there are about ten swarms here, although the number of swarms that emerge depends upon the level of threat. Animal spells of R: Voice, D: Conc, T: Group will affect one swarm (assuming that Penetration Total is sufficient), and at one magnitude higher (extending T: Group tenfold) the island can be temporarily cleared of the mice."
This character (my wife's maga) already had DEO at Individual. She cast it; a single mouse-demon died. It was clear that another plan was needed!
The character is also an expert (for a mage-just-out-of-apprenticeship) at Muto Vim spells, at concentrating on multiple spells, has Cautious Sorceror, etc. So, boosting her own spells in a dangerous aura is absolutely what the character does. They also want to come back and have another pop at the island as soon as possible but they don't have much of a library, so can't just pluck useful texts off the shelf. The saga is very political so they can get lots of things from other covenants but it's all at a cost and they are being cautious about over-committing.
While there are a number of ways to approach this, the over-riding principle for us is: "What would my character want to do?" which is often very different from "What is the most effective way to achieve this within the rules of Ars Magica?" Gretel ex Merinita (ahem and definitely not of another house that shouldn't exist) likes to mutate spells so that's why the particular focus on that aspect.
All of that said, the excellent suggestions thus far on ways to approach the problem are really useful to me as it helps to educate me on the rules and reveals other options that I've forgotten or missed - I'm running the saga, the three players are all very new to the game and don't have the time to immerse themselves in the rules/forum to the degree that Ars Magica sometimes seems to need. Advice that I can give to them is something of a necessity (the poor fools!)
I'm definitely going to recommend that she look into the Mastery options, though, as that will help her to get better at her chosen schtick.
I stand corrected, what you refer to was posted in response to a question I had in 2014 or so. Someone posted something David Chart supposedly wrote. And that is one way of solving it.
Personally I'd rather have Muto Vim be an exception, so you can cast it in the same round as the target spell.
But my point was, that if the first solution was true, you could only cast the DEO every other round, because you needed to cast the MuVi inbetween. So that's 10 demons destroyed per 2 rounds. Comparing that to DEO T:Ind with Mastery (multicast) only requires Mastery score of 4. You would have to roll 5 times each round, but with a reduction in Botch dice due to Mastery.
I apologize for going off topic, however it matters how Muto Vim works, because this affects the method proposed and whether it is better or worse than others.
Personally, I play the two-round method unless you choose to fast-cast the MuVi spell since you are explicitly limited in the number of non-fast-cast spells in a round and nothing in MuVi says it breaks that limit.
Regardless, I still think Spell Mastery is the way to go. No, don't go for Multiple Casting (well, you could eventually). Rather, using T: Group and getting Penetration with Mastery 2 gives you nearly as much penetration as might be achieved by the Pair spell, and it gives you so many other benefits along the way. One of those is not worrying about the 1/2 rounds or simultaneous casting issue. Others are botch dice and concentration.
I found the ordering of target unpleasant, with Part between Ind and Group, until reading Hedge Magic and noticing how Gruagach have non standard spell parameters, and have to invest in extra magnitudes either to go up or down the default parameters. Then I thought that Personal / Momentary / Individual was not the absolute minimum, just the default setting, so the actual ordering for T would be Part (+1 Mag), Ind (+0 Mags), Group (+2 Mags)...
T: Pair with +1 Magnitude fills the gap between Ind and Group, not actually sharing anything with Part besides the Cost. And magi with FFM would be able to switch from Ind to Part, narrowing the target, or Pair, widening it, from Pair to Ind and Group and so on, but not from Part to Group, not because it seems extremely awkward, but because that would be not one step away, but 3.
Regarding spells which actually do nothing except when modified somehow, I totally agree with that. I guess you can see hermetic magics as the combination of parameters and effects; the vast majority of these won't actually make anything or work or be of any use at all, being the background noise of the hermetic magic system. Working spells would then be like bright spots between all that random noise. Which actually makes a nice concept for an odd Bonisagus or Criamon, interested in the distribution of these bright spots between the noise; because if there is some order in there, well, he could even discover spells (or effects, or parameters) just because the pattern and the theory predicts them, no matter if he have the arts or the intention (or maybe a clue of what they do) to invent them.
Oh boy. I see this as good way to add T: Pair (first success for this magus!), and escalating into a Mystery Cult of Hermetic Topology.
T:Pair fixes such a glaring issue with FFM and MuVi that I would allow research and networking to dig up enough old Lab Texts to provide most of the breakthrough points towards a Minor Breakthrough that would make it a standard parameter.
(and as a player I would probably argue for a house rule that this has been done already)