Hey! I'm new to Ars Magica and this forum. It seems like a small community, but I've had fun reading a lot of the threads here.
My first character, which I've been playing for a couple dozen sessions now, is Celestis of Merinita. She specializes in Muto and Rego Corpus, and has a Major Magical Focus in Human Body Modification, which she uses to make people beautiful, and also give herself weird powers.
So! I'm curious how a few things work for Muto Corpus, and would appreciate if anyone more knowledgeable could give me some guidance on the matter.
Are the guidelines for granting soak with Muto Corpus intended to scale beyond the table in the core book? At level 5, one soak can be granted, increasing by one soak per 5 levels until 25, and then it stops. I can't tell if this was to save room/wordcount, or if five soak is the current limit of hermetic understanding.
Is it possible to grant something like superhuman strength with Muto Corpus? For example, if one made a spell to grant temporary gigantic size, could the spell also grant appropriate strength for the size? Or is that solely the purview of Creo Corpus, which can only grant up to peak human strength? I found a MuCo spell in... I think it was Magi of Hermes - that lets the target move as fast as a horse. So, there's a precedent for superhuman speed.
What are some neat Muto Corpus effects Celestis could invent in the future? The GM approved a "Grant Major Ability" guideline at Base Level 20, but I've only used it so far to make a superhuman beauty spell, and a spell that grants the target the ability to retract their limbs seamlessly into their body (and extend them again later).
I think it’s reasonable to say +5 is the top for a MuCo spell, after all, in MuAn, a parallel spell, DOUBLET OF IMPENETRABLE SILK, states “ The magic does not make the armor better at absorbing shock, so this +3 bonus is the limit that can be bestowed without changing the material into something completely different.”
What you could do is include a Terram prereq, so that your flesh transforms into iron to a certain extent , for example, and this would reasonably push the max Soak bonus considerably higher.
Please be aware that formulaic spells above 50 do not exist, they are now Rituals, by default, although you could create a magic item that provides very high effects, bypassing this limit.
Whether +5 Soak is the limit or not is up to the troupe/SG. We necessarily know the guidelines in the core book are incomplete, as shown by the core book itself. And even in the case of increasing guidelines like these we have examples that go beyond what the core book lists. But that does not guarantee there is no cap to any specific one of them, which is why it's up to the troupe/SG.
ArM5 is not a game like e. g. old Champions, where many rule concepts were meant to scale freely.
New Hermetic guidelines are introduced in many later books. They always took quite some thoughts, and sometimes later rework also. Don't be hasty here, as you can risk your saga.
For inspiration on general MuCo / MuAn spells and how to test their limits in a saga, Marcus of Criamon (Magi of Hermes p.79ff) and his development over some 45 years is useful. It comprises a yet unachieved Breakthrough Hermetic Metamorphosis and many spells enhancing animals and people, sometimes in gruesome ways.
I can't recall where it is, but increasing in size grants an automatic bonus to strength and an automatic malus to speed per size increment.
EDIT: It is certainly in Hedge Magic, in the gruagach spell section: +2 Strength and -1 Quickness per point of size increment. I did not find it in the core book though. I know it is in other books as well, not just hedge magic. In the core book preternatural growth and shrinking does not list any change in your characteristics.
No general rule is given concerning the effect of a Size modification.
Giant blood gives +2 size, +1 Strength and Stamina, Large gives only +1 size. No loss in Quickness.
But someone could have a point in saying that it's a natural giant-blooded character, not one artificially changed in Size.
The Muto Corpus spell to change Size doesn't give any hint about other effects of this change.
In the description of the gruagach Shape power (Hedge magic, 68-69), you have a spell described as giving +2 Strength and -1 Quickness for each Size point gained (so it's +5 Size, +10 Strength, -5 Quickness).
So I guess you have a choice between those two rules, or could try combine them as you can.
Actually, we do have a hint now, and it's not an automatic +2 Str and -1 Qui. Take a look at Assume the Stature of the Giants of Eld (MoH p.49). Preternatural Growth and Shrinking wasn't explicit about anything, but this spell is. I suspect the general variation really started with the core book's V&F like Large and Small Frame.
Probably the matter - unlike Creo or Perdo magic, Muto doesn't directly improve or devalue its target, but rather adds qualities to the any existing ones so what the caster is effectively doing is giving the target's skin and flesh the damage repelling properties of wood, stone, etc for the duration of the effect.
As others have noted, Size increases automatically improve Strength by extrapolation of the rules introduced in HoH: MC. Alternately, depending on what you want to use said Strength for, it might also be doable by incorporating a Animal requisite and becoming a powerful creature such as a bear or oxen.
Some suggestions are listed under the equivalent Muto Animal guidelines and includes stuff like granting a horse claws, fangs, and scales, the power of flight, or the ability to breat fire, etc. Ultimately, however, it's the sort of thing the player and SG are supposed to discuss as is the case with many things in Ars Magica.