So, when casting e.g. Wizard's Boost (Ignem) on a Pilum of Fire, you need a Concentration roll of 9.
But if I have Imperturbable Casting Mastery I add Mastery to Concentration.
But which of the two spells do I need this Mastery with to use this? Either one? Both? The Pilum, because it is this spell I risk failing with because I also cast boost? Or the Boost, be Use it is this spell I risk failing when I try to cast it at the same time as The Pilum?
I would never presume to let it stack if both spells in fact have Imperturbable Mastery
The Concentration Table on ArM5 p.82 requires to concentrate on an existing spell while casting a new one.
This implies, that Imperturbable Casting needs to be a Mastery Special Ability for the Pilum of Fire, to apply in the case that its caster then casts a Wizard's Boost (Ignem) onto it, too. Yes, by Muto Vim Guidelines ArM5 p.159 both are then cast in the same round, but it is the Wizard's Boost (Ignem) that needs to be timed after the Pilum of Fire by the same Guidelines.
One Shot, a lot of that second paragraph is off. I'm on a phone, so I cannot search and copy/paste things until later. But the MuVi spell comes first, and the core rules do not say you get two regular spells in a round. The clarification was that round 1 is MuVi and round 2 is the other spell. Also, I don't think the wording of the Mastery option perfectly agrees with your statement.
Also, it appears to be impossible to cast two spells in the same round, unless one of them is fast cast. I'm fine with Round 1 MuVi spell and then Round 2 Spell to be boosted, though, otherwise meta magic like this would be next to impossible, rather than just really hard.
For a more precise grasp, first have a look at this, too:
So we see (by your quote above), that Pilum of Fire has still to be in the state of being cast, while Wizard's Boost (Ignem) is cast onto it. We see also (from the errata I quoted) that the Pilum of Fire needs to be available as the target of the Wizard's Boost (Ignem). As we see by
, you can indeed on your own cast both spells, so that Wizard's Boost (Ignem) can affect Pilum of Fire. This can only happen if Pilum of Fire, while still being cast, provides the target for Wizard's Boost (Ignem). And casting of Wizard's Boost (Ignem) cannot begin before its target is present.
[Also, given that Pilum of Fire is a D: Mom spell, it would be gone after it is cast - so is not only (by your quote) no longer targetable, but it is also no longer exists as a target.]
Together this says, that Wizard's Boost (Ignem) needs to be cast in the same round, but a starting little later, than the Pilum of Fire it shall affect. This fits well with the rules about one magus casting Wizard's Boost (Ignem) onto the Pilum of Fire cast by another.
I can imagine a troupe requiring that a MuVi spell be fast-cast even to affect your own spell, though.
Work out the logic behind making the Muto Vim spell come as the second spell or have a duration longer than momentary, when the text explicitly says "if the casting time [of the other spell] is longer for any reason..."
Take your time and come back later. No need to hurry.
By the Muto Vim Guidelines box on ArM5 p.159, Mastery with Special AbilityFast Casting is needed for every non-cooperative casting of a MuVi spell onto another's spell - so it is anyway often a typical choice.
So, that just says that your MuVi spell, once cast onto its target (the spell being cast), has to affect it for all its further casting time. Still, the spell is explicitly defined as the target, not its casting.
Such logic only leads me to conclude that Muto Vim magic is therefore impossible, except with two magi in cooperation, or as a fast cast with someone who isn't cooperating. One can never use Muto Vim on one's spells (or even items) for the following reasons
You cannot affect a spell that has been cast with Muto Vim.
You cannot cast two spells simultaneously in any amount of time, one spell has to be cast first.
And, in your interpretation (which I can see to some extent), Muto Vim spells fail if they don't have a valid target if cast first.
Why do you believe you can cast two spells simultaneously?
It's not the Concentration Table on page 82, it's not listed in the Situation part of the table. And I would dearly wish it to be, so I could have my Lightning caster do something riduclously hard and foolhardy (she has Poor Concentration) and cast like 8 lightning bolts at once.
Simultaneous spell casting is also not indicated in the Muto Vim guidelines.
It is implied there for MuVi spells changing the caster's spells.
It was already implied in the unerrataed ArM5 1st edition, that the target of a MuVi spell was the spell to be changed:
It is also stated there - as already quoted - that you can only MuVi spells while they are being cast.
So you could ever only affect your own spells with MuVi if you cast the MuVi onto the target spell while you were casting that target spell. And these MuVi spells always required only an Int + Concentration roll of 9+ for this: they always were special in many respects.
And this went through two bouts of errata, was never changed, and holds still true.
There's a stronger implication that the Muto Vim spell is cast first, especially in light of Wizard's Communion of D:Sun to affect rituals.
There is a chicken and egg problem here, I don't disagree. I can rationalize it away as a Muto Vim spell creates a matrix for receiving the spell that it will target. I don't have to deal with the concept of being able to cast two spells simultaneously, in this one single instance, nor do I have to accept the idea that if the Muto Vim spell is cast second that it affects a spell that has already been cast, but it only works on your spells and no others. It is largely an aesthetic choice, granted, and it's also closer to my reading of the RAW.
If you can cast two spells simultaneously there is absolutely no reason to have a D:Sun version of Wizard's Communion, because you just time the casting and roll your Int+Concentration roll of 9 to get it off while putting the finishing touches on the ritual.
Yes. Wizard's Communion isn't errataed there. And it doesn't target a spell, but a group of magi about to cast a spell, joining them in Communion. So why is a D: Sun Wizard's Communion relevant for your argument?
Because Wizard's Communion (WC) is a Muto Vim spell. Because the language in the errata, regarding D:Sun is because of WC and the need to have a version that is running for as long as the casting of a ritual...