OK, as promised here are a few of my ideas for systems that I think could successfully represent the Cult of Mercury's magic. Obviously, some have been given a tad more thought than others, but I'm not completely satisfied with any of them and would love to hear other people's suggestions for improving them.
As briefly mentioned in Step 1, this method was developed by taking a synchretic approach to combining Fenicil's Rituals (HoH: TL) with Vigilo Magic (RM).
Under this method, each cult possesses a certain number of Schools (not unlike those used by the Augustan Wizards) representing particular aspects of their patron deity (for Mercury, Herald, Shepherd, Thief, etc). These schools would be used to design spells using a set of guidelines created using the same method CJR used for Vigilo Magic (Hermetic level x 3 or Magnitude+4 x 3, whichever's lower) though naturally these might be adjusted up or down a few magnitudes for specialized traditions. The schools themselves don't contribute to Casting Totals, but do confer Magic Defences and limit how much Vis can be used per season per ritual participant.
Whenever a Mercurian learns a new spell, he gains a Spell Mastery ability of 1 in that spell which is then used in the casting total for the spell. Most other aspects of Mercurian spellcasting are as per the Augustan model except that Mercurians have the option to cast cooperatively as per Fencil's Rituals and have various secret methods (Mysteries) for reducing Vis costs.
One other ability requires special mention: Communion. Communion is the Mercurian equivalent of Ceremony and works in much the same way except that it automatically increases to remain equal to the caster's highest Spell Mastery ability.
[i]Example 1: Justin seeks to cast a Mercurian equivalent of 'Chirurgeon's Healing Touch' (Base 15, +5 = Level 20). The spell has an EF of 21, +3 Touch for a total of 24 and thus costs 4 pawns of Vis and takes an hour to perform. Note that the Vis cost and casting time are identical to that of a Hermetic magus.
Stamina 1 + Spell Mastery 4 + Aura 3 + Stress Die 6 + Communion 4 - Participants 1 = 17; almost enough, but not quite. With a few more spell mastery points from his fellows, however, he might make it...[/i]
[i]Example 2: Justin, along with a small group of his disciples, are asked to summon up a former Emperor's ghost to advise his successor (Base 15, +4 AC, +2 Ring = 45) for a an EF of 39. The ritual requires 7 pawns and takes 1 hour, 45 minutes to perform. A Hermetic would require 9 pawns and 2 hours, 15 minutes.
Stamina 4 + Spell Mastery 11 + Aura 3 + Stress Die 6 + Communion 18 - Participants 6 = 36; this ritual goes off without a hitch. The Emperor rewards Justin handsomely.[/i]
Pros: Low-level spells proportionally more difficult than high level ones, Spell Mastery is extremely important, guidelines are fairly easy to calculate, number of spells known is kept low by need for Mastery.
Cons: Group rituals may be too powerful, low level rituals are costly in Vis, School guidelines must be compiled, Learning new spells isn't hard, Hermetics with Mercurian Magic are superior in every way.
This method uses a less in-depth model, but one that I think might work better under some circumstances under it, the Mercurians can't invent spells at all per se doing away with the need for Schools or Arts. Instead, any spells or enchantments exist as Lab Texts passed down from Antiquity.
Spells are learned automatically with a value of one can be improved with experience according to the abilities scale as per Spell Mastery. Ease Factors for the spells are equal to (Hermetic value x 3). Spells can only be cast, however, if the group's combined Wizard's Communion spell total is equal to or greater than the spell being attempted in which case the Level is divided by the number of participants to achieve a Casting Total.
Naturally, casting time and Vis costs would be identical to Hermetic equivalents though they would benefit from the effects of the Mercurian Magic virtue.
[i]Example 1. Chirurgeon's Healing Touch is a Level 20 spell for Hermetic Magi so it's Level 60 for Justin and his comrades. There are 4 casters and each knows Wizard's Communion at Level 20 for a total of 80. This reduces the EF of the spell to 15.
Stamina 0 + Spell 5 + Aura 3 + Die 6 = 14; this ritual goes off healing the target. Note, however, that Justin would have failed completely on his own.[/i]
[i]Example 2. The Ghost summoning above (Hermetic Level = 45) has a truly impressive Mercurian Level of 135 so Justin calls together 8 of his most trusted allies to participate. Their combined Communion of 150 is sufficient to reduce the EF of casting to just 15 (again).
Stamina 0 + Spell 4 + Aura 4 + Die 6 = 14; the ritual is successfully cast yet again![/i]
Pros: Easy to work with, strongly emphasizes low-level spells for individual, high-level rituals easily cast as part of a group, implements Wizard's Communion.
Cons: Single casters are all but helpless, even small inexperienced groups can be very powerful, no power progression in spells learned, lots of spells can be learned.
Under this method, Mercurian Wizards possess a series of virtues that work like Folk Magic in TSE.
Basically allowing the wizard in question to use Magic Lore to perform certain free-form effects associated with particular Hermetic guidelines. Thus, Mercurius Fur might be associated with Perdo Imaginem or Muto Imaginem effects while other virtues grant other guidelines.
Such spells, in accordance with the rules for Folk Magic would require a minimum casting time of 15 minutes, but not necessarily any vis. Casting Totals are necessarily low, but not quite to the point of uselessness and can be enhanced for group casting by any of three methods.
A. Ceremony - adding Stamina, Magic Lore, and Group Bonuses of extra participants
B. Communion - Dividing spell levels by # participants.
C. Vis - Casting Totals are boosted with Vis, but each participant can only spend a single pawn so large groups are required for large-scale castings of any sort.
Personally, I'm especially fond of C, but it might work best paired with A or B. In any case, if this option were chosen, it would require a rather poetic interpretation about the nature of spells in which we attribute concerns about the Cult's repertoire to the fact that any given Mercurian is unlikely to be able to cast more than a handful of very simple spells on their own.
Pros: Low Casting Totals, scales well, vis intense, tradition is useful and fun at all levels.
Cons: No Mastery, awfully close to spontaneous magic, dividing die rolls is annoying, possibly too costly.
Alright, so that's a lot to take in, but I look forward to your critiques as well as hearing about your own suggestions for mechanical systems that could represent Mercurian spells in Ars Magica.