What happens if you summon a Theurgical spirit to Cast Wizard's communion effects?
Would this let you get the advantages of a circle of wizards without the wizards?
One definite advantage is no botch chance on the Wizards Communion, and no need for the Concentration roll since you're not technically casting both spells yourself.
Invoke the Spirit of Legion MuVi20
Summons a spirit of might 20 will cast Wizard's Communion Lvl 20 for you.
(I'm inclined to say yes, especially since Hermetic Theurgy is seen as underpowered, but I 'd like a 'reality' check)
Okay, if it does work, can a magi then invent multiple versions of this so that the 'circle' has more members?
Invoke the Spirit of Legion MuVi25@diameter Invoke the 2nd Spirit of Legion MuVi25 Invoke the 3rd Spirit of Legion MuVi25
Etc?
Yes, if you want to commune with 5 spirits, you need 5 different spells, and they need to stick arround long enough for you to call them all... But yes, this is one good use of spirit of spells...
Absolutely, it is one of the many benefits of Hermetic Theurgy. The only potential problem to look out for the DM/Troupe fiat on the following:
"The Spirit casts the spell and then departs"
Duration on Invoke the Spirit spells determines for how long the spirit will stick around before casting it's spell. Exactly what happens in case of Concentration spells, or spells that require their continued presence and participation is unclear.
The standard variation of Wizard's Communion works without issue. Single-round casting and you're enjoying all the benefits you need. Wizard's Vigil on the other hand...
If memory serves, the errata essentially says a MuVi spell needs to last for the casting time, not that it needs to be cast and concentrated on during the entire casting time. So for an invoke the spirit of Wizard's Vigil spell, you would be instructing your spirit to cast vigil as you attempt the ritual, and hopefully you were smart enough to consider where the sun is prior to casting so the spell lasts long enough. There should otherwise be no problem with that spell conflicting with the description in hermetic theurgy.
Spells with Finesse and concentration... Yes, you're quite right.
The spell lasts long enough, sure. But you might need all the casters around for the entire casting time to have them count as participants for reducing the casting level of the main spell.
Not necessarily the caster(s) as such, actually. But the people participating in the Communion/Vigil.
Can they be considered to be participating if they leave as soon as it has begun? How can they combine their power if they aren't even there when the main spell goes off?
Wizard's Communion is a remnant of old Mercurian practices, which typically required many people working together.
As I see the spell it is not a necessity by RAW that all the WizCom casters have to stay there for the entire casting time of the shared spell.
It is more of a logical deduction based on what we know about rules and History of Mythic Europe.
But it is an enormous contradiction how the Mercurians were able to work together during this communion WITHOUT Parma Magica. The Gift should have blown their minds.
One can resolve that contradiction with the hypothesis that the Mercurians did their communion "one-by-one": one came, he cast a Wizards Vigil, went away, came the next, she cast the next Vigil, went away... And so on.
That looks like an exaggeration. Compare ArMDE 02-Order of Hermes:
One of the few even partial successes in this regard was the Roman Cult of Mercury. Its members rarely met in person, communicating by letter or messenger except when they needed to gather to perform the great rituals that gave the Cult its power. Further, the rules of the Cult specified that anyone who killed a Cult member should be hunted down and killed by the other members. This rule was enforced quite strictly, greatly reducing the incentive to pre-emptive strikes. Finally, the Cult of Mercury had access to a number of magical effects which could be learned relatively easily, even by someone who already knew many of them. This gave the wizards some motivation to share knowledge.
Nevertheless, suspicions grew within the Cult that some members were hoarding knowledge, ready for a major strike against the other members. Soon after the fall of the Western Roman Empire, the Cult tore itself apart in internal conflict, leaving isolated wizards scattered across Europe. Magic entered a dark age, from which it emerged with the foundation of the Order of Hermes, more than three centuries later.
It's not a general rule applicable to everything Mercurian. See for example the scrying pool (Mythic Locations 98) with its peculiar mechanics of stay if you want, leave if you want, come back if you want, as long as someone keeps going... I suspect the greater rituals of fenicil works something along the same way rather than everyone meeting for 8h a day and then the casting stops for the night and resumes the day after.
Although Wizard's Communion and Vigil don't perfectly fit into hermetic theory and has some weird side effects like it can't be sponted, that doesn't mean they don't, per se, obey the regular rules of Muto Vim. And neither communion, nor vigil say the caster needs to stick arround. Likely, if this was required at home point, it no longer is, as there is no general requirement to stick arround after casting a D:Sun MuVi spell to boost a ritual - and when spells have an exception to the regular rules, they say so. It's still an hermetic spell by now, even if imperfectly integrated.
This effect is a special feature of how this ritual was originally designed to interact with Wizard’s Communion; these rules do not apply to other spells.
So this is a rule for a non-Hermetic feature of the adventure The Fallen Temple of Mercury, not a general property of Wizard's Communion / Wizard's Vigil.
Especially, every magus who has joined the casting of a ritual by Wizard's Vigil, that then got botched by the main caster, does (ArMDE 09-spells)
gain Warping Points and must check for Twilight if they gain two or more.
That would be extremely odd if such a magus would have left the casting process already before the botch happened to the ritual's main caster.
The imagination of magi 
