Let's take, for example, a Mercurian Magus casting Aegis of the Hearth. He knows the Aegis at Level 30, so he's got the Communion at Level 30 as well. Since he needs 60 Levels of Communion, he gets two Sodales who know Communion at Level 15 to lend him a hand.
So, the level requirement for the Communion is fulfilled. Three Magi participating means the Magus casting the Aegis only needs to make TN 10.
Who needs to cast the Communion? The Magus casting the Aegis, the Magus knowing Communion at the highest Level, or every Magus participating in the Communion?
What's the Vis Cost of the Aegis? 6 for Level 30, with 3 if all Participants are Mercurian Magi? Or 2 for TN 10, with 1 if all Participants are Mercurian Magi?
Is there any benefit in getting 32+ Penetration on a Level 30 Aegis, given that the Aegis cannot stop anything with Level / Might >30?
All the magi need to cast the Wizards Communion at their levels and succeed, and one Magus needs to cast the Aegis.
I'd say Vis cost is standard, or standard for the Aegis caster only , they might have virtues which reduce Vis cost. Virtues of the other magi shouldn't factor in.
In RAW I don't think there is a penetration benefit, some sagas vary.
Lastly the Wizards are adding botch dice for no benefit in your scenario by using a Communion.
"However, the target number for the spellcasting roll is the spell’s level divided by the number of magi participating in the Communion."
The spell level remains the same, but the Casting Total is compared to a reduced value for success and penetration.
"Penetration totals for magical creatures are reduced by the same amount."
A Might 35 creature would have -15 Penetration on its powers if you get your Penetration to 35+.
Another complication of using Wizard's Communion for a ritual comes from the errata, which states that a Duration: Sun is needed for Rituals instead of Momentary. Great for shared Formulaic spells as is, won't work for Rituals unless you house rule that its ok. Which I think is common enough.
Don't forget that creatures trying to enter the Aegis might benefit from the local Aura (Magic and Faerie creatures), so a Magic Might 29 creature could have +33 Magic Resistance if the local aura is Magic 4. Some creatures also have Form Resistance, and Familiars share their magus' Form Resistance in place of their Magic Might.
Given that it's a Mercurian descendant spell and they were all about ritual magics, yeah, the spell was badly designed from the get go. We always just change the duration to 'special'.
The Wizard's Communion in it's current form has been with us not only long before the current errata (or guidelines) for MuVi, but also for editions longer than the Mercurian Magic Virtue and certainly longer than the current version of Ritual Magic. Wizard's Communion has been with us (largely unchanged) since the first edition. And it used to do exactly what it was designed for.
With the current Errata, perhaps Mercurian Magic should have been errata'ed to grant not the Wizard's Communion, but the Ritual Communion, or whatever people want to call it.
I think Through the Aegis has a version, which I'm too lazy to look up right now.
Look at the 4th edition version of Wizard's Communion: R: Reach/Near, D:Spec,T:Group. Duration: Special. The mechanics of it were different too, but the spell was obviously overhauled for 5th edition. I'd argue the mechanics of it working are better in 5th, but the design of it didn't take ritual magic into consideration at all, and that was an error.
If you've got 1st/2nd/3rd editions handy, I'd love to see what those versions are.
Well it did take it into account, but then the Muto Vim guidelines were corrected through an erratum.
Ritual Communion, as I like to call it is a Sun duration spell, and it adds 10 levels less than its level to a communion. 10 less levels, to account for the +2 magnitudes from D:Sun.
It is probably best for all players to remember this, and not factor the aura into their desired penetration totals for anything, period. If you want to have 30 penetration, go for 30, not for 25, when in an aura of 5, or some other number. It's going to ruin your day when you can't toss a BoAF or IoL at the might 30 dragon terrorizing your covenant, and you can't penetrate because you stopped when you got to 25, because the aura takes you to 30, forgetting that it takes the dragon to 35.
Also, if you go into adverse auras, the differences become much more pronounced, and that might 30 faerie will still win over your 30 penetration spell, because the aura adds the full effect to his resistance, while only half the magnitude of the aura is added to your casting total. Infernal auras are much worse than faeries, since the infernal aura subtracts its magnitude from the casting total, and adds it to the creature's resistance, and of course, the divine aurs are the worst, doing 3x magnitude from the casting total, and adding the magnitude to the resistance.
So the MuVi rules were flawed (thus errata'd) and Wizard's Communion was built using those flawed rules - thus Wizard's Communion was flawed. Now you need an entirely different spell for Mercurian Magicians to work together on Ritual spells like they're supposed to. So do you house rule Mercurian Magic or Wizard's Communion?
It's just plain easier to house rule Wizard's Communion to have a 'Special' Duration and have done with it. It also eliminates the need for a Diameter version of the spell for magi with Slow Caster.
R: Reach no longer exists. Neither does Near. Near was largely replaced with R: Voice.
D: Special: Good catch, I'd missed that, as I was focussed on the text of the spell. But it matters little, since there was not the requirement that the MuVi spell lasts for the duration of the spellcasting.
T: Group: Same as current.
But let's see, the varions versions:
1st ed (2nd printing): Wizard's Communion: Gen, Reach, Inst. Text substatially but not entirely identical to the current version.
2nd edition: Wizard's Communion: Gen, Reach, Inst. Text appears to be identical to the 1st edition (I do not have them side by side).
3rd edition: Wizard's Communion: Gen, Spell Focus: None, Reach/Near, Inst.
4th edition: Wizard's Communion, R: Reach/Near, D: Spec, T: Group (Target as a parameter, as well as actual guidelines, were introduced in this edition, but used far less consistently in my experience).
5th edition: Wizard's Communion: R: Voice, D: Mom, T: Group (again, Reach and Near were both eliminated, Voice must've seemed most appropriate. Text largely identical, but slightly re-written because casting from text in no longer a thing.)
Mercurian Magic first appeared (to my knowledge) in the 2nd edition 'scenario' The Tempest, on p. 15, with a almost completely different content.
I think it re-surfaced in the 4th edition WGRE, which I do not happen to have handy, so can't look it up for you there.
And again, the problem isn't really with Wizard's Communion as written.
The problem was possibly with the MuVi guidelines saying that aMuVi spell had to had a duration atleast as long as the spell it was to affect (ArM5, p. 159), which was later changed with the rather substantial (and to my mind superior) MuVi errata referenced above.
But more likely, it was with the Mercurian Magic Virtue, which had already been substantially altered from earlier version, and should probably have been errata'ed to give not a Wizard's Communion (used to cast simple formulaic spells, eg. Circular Wards) but the Day of Communion, found in Through the Aegis, p. 137, top of 3rd column, and possibly other places in this text as well, which is designed for use with rituals, under the current errata.
For you, maybe. I buckle every time I have to use a 'special' parameter, considering it the sign of either shoe-horning in something from an earlier edition, or a lazy spell-designer.
Before the errata, it was presumed that Momentary MuVi spells were sufficient to cast rituals; so WC was sufficient for Mercurians.
There's multiple moving parts on this issue (MuVi guidelines, Wizard's Communion, Mercurian Magic). Obviously there were large problems with the MuVi guidelines, thus the multiple errata over the years. The errata created the issues with WC (new spells required) and Mercurian Magics - WC as written isn't useless for a Mercurian, but Day of Communion is the one they want. Heck, Day of Communion is the spell most magi would learn first - to better their Aegis - yet Wizard's Communion is the one sitting in the core book, and that IMO is problematic.
Changing WC's duration to 'special' might be 'lazy' spell design, but it's a simple change that doesn't screw around with MuVi guidelines or Mercurian Magic in unintended ways. And it IS a legacy spell, after all.
As an aside, the way the MuVi guideline errata (2nd printing) reads, it is really a kludge to get around Muto Vim spells not having to last as long as the spell they change in order to have the change last the entire duration of the spell.
Maybe 6th edition will make MuVi guidelines more consistent (fewer exceptions). I think a MuVi spell should have to last the full duration of the spell it's cast upon or the spell will change back - just like any other Muto effect. Ritual spellcasting should have the concept of multiple participants built into it - just like adding Artes Liberales and Philosophae is built into it. Make Wizard's Communion the quick and dirty spell version of ritual group casting, great for Momentary duration spells but really hard for Moon Duration stuff.
It just screws around with the basis of Hermetic magic. Changing the the spell that Mercurians get to D:Sun, and having a reduction in power levels to account for the magnitude necessary for D:Sun is much easier, and doesn't require a special duration being created in contravention of the entire spell system.
I'm more than happy with WC as is and requiring a Sun version for Rituals. That's no different from the other core rule book examples which only have a single instance of an effect. What would have been handy is a extrapolated explanation somewhere on the official stance with the errata.
Aside on the Range :Voice for WC, and if it could be recreated as R:Touch? (Asked above).
My suss is that it needs to be Voice so that the WC can participate in the changing of a spell being modified cooperatively, which is what the guidelines say. That then strongly implies that each wizard also needs to cast WC successfully to change the spell, which isn't clear.
Some are also attached to virtues, like Atlantean Magic or Holy Magic. No system is perfectly able to create mechanisms for every desired option; some things need custom allowances. Wizard's Communion gets its own base guideline - with Voice and Group it's getting +4M effectively for free, while all the other MuVi guidelines are (level + 1 magnitude).
If we really wanted to get technical, I'd think a MuVi spell should last as long as the spellcasting for the target spell takes. That's reasonably well defined enough for my tastes.
As it stands, magi casting "Day of Communion" just throw it out at the start of the ritual and then they can effectively leave? I don't have Through the Aegis yet, but this seems to be the implication. If this is so, it's very anti-climactic. Making the spellcasting of Wizard's Communion last as long as the spellcasting of the spell it's altering is both thematic AND balanced, because it demands a time commitment equal to that of the spell being changed. In this case, longer duration is a detriment.
If I was in an Ars Magica campaign and the storyguide required Day of Communion, I'd be okay with it, though I'm pretty sure I'd walk out of rituals after my 6 seconds casting it. Can anyone post a summary of that spell?
There are numerous freebies in the base spell guidelines, Wizard's Communion isn't the only recipient, although it may be the spell that gets the most benefit.
Ritual spells, IMO are anticlimactic. It was part of my thread of rethinking ritual magic.
While ritual spells have a time requirement of 15 minutes per magnitude, in active play this tends to get lost or ignored, because Ars goes from the season or year (advancement) or to the story, which is much more of an in the moment experience. Aegis is little more than a bookkeeping exercise, and other rituals will often get handwaved. And David Chart's comments to the contrary (not errata, mind you) the risks of casting rituals, IMO are way too high: Magnitude+1 + other botch dice considerations. One would be insane to cast a powerful ritual under stressful conditions. The system is poorly designed to handle the importance of rituals, IMO. So, that a spell like Wizard's Communion seems anti-climactic to you, I'd place that on the nature of rituals, and not on the ability to commune or not.