While the Cult of mercury do not need game mechanics, my brain went there. I wanted to think of why they succeeded even though the gift suggests they shouldn't, and why did they collapse.
Basic Ritual Provision.
The cult of Mercury cast Ritual spells that generally have small bonuses over a large area. A casting circle can only have 1 of any given ritual active at any time. if a new ritual is cast, it cancels the effect of the old ritual. So multiple rituals fine, copies no.
The casting circle comprises of a core of gifted magi. The first magi must have a mercurian ritual skill of 1 to be in the circle, the second magi 2, the third magi 3, etc.
The outer circle comprises of trained members who do not need the gift. The maximum outer circle member count is 10* the highest mercurian ritual skill of the inner circle.
Each ritual casting is it's own skill. Anyone involved in the ritual must have a minimum skill of 2. The casting total of each ritual is:-
Sum of Inner Circle's skill + sum of half of each outer circle member, round up + aura mod
This will often get casting totals around 230+ on the premise of 10 core members, and 100 outside circle members, which seems reasonable.
My thinking is there is a focus on getting Mercurian Ritual up to 8,9,10+ and the other ritual skills rarely past 3, as the outer circle pumps up the numbers.
Each ritual can be modified for a different duration, target and effect. This is done at the start of the ritual, and they are not separate rituals requiring different skills. The modifiers are added together, and then the base spell level is multiplied by the sum of the modifiers.
Duration.
Event - a short term event such as a skirmish, a harvest festival, a parade, etc. No mod.
Year - +2
Target.
Casters - Inner Circle Only No mod.
Group - the casters and a group of people no larger than the inner circle number.+1
City - +3
Empire +5.
Effect. +1/+2/+3, modifier +1/+3/+6.
If One wanted a +3 bonus effecting the entire empire for a year, that would be a +13, meaning the base level is *14.
Cost -
Every 2 levels of the spell requires a fatigue level to be provided which is averaged over the circle. On a botch or if the ritual fails, every person inner or outer circle makes a skill check against the relevant ritual target 9 (modified by aura). For each point failed, a fatigue level is spent. a botched makes the damage wounds, not fatigue.
A botched ritual has a lot of unconscious person, and if unlucky deaths.
Aura Effect
Magic and Fairy Auras add their bonus to the skill level of each inner circle members skill, with a maximum of the relevant ritual skill. No effect on outer circle.
Divine and infernal - negative effect equal to the aura, affects inner and outer circle. Extra botch dice equal to the level of the aura.
When the cult begun, these auras were very rare. The first time the large area they used transitioned from a Jupiter/Magi/Faerie to divine aura, everything went poorly. This justifies their disappearance of the cult.
Rituals
All rituals are smart enough when cast on a large area to work out who is or is not working in the interests of the cult and their patrons.
Blessing of Mercury (lvl 15) - Everyone travelling in the target area who is a defined friend of the casting group) , gets effect bonus on everything travel related. Combat rolls vs bandit, animal handling roles, luck roles to see if a wheel breaks, navigations, building roads, etc.
Blessing of Saturn (lvl 30) - Reduces the ravages of time. All aging roles have a bonus of effect level. Normally applied to the highest senate members and the inner circle of the cult.
Blessing of Pax (lvl 30) - Gives a bonus (or counteracts a penalty) when conducting peaceful interaction with people. If cast at level 3 removes all penalty of the gift, not just the -3, but the general ill feeling the gift provides. {this ritual is how they manage to not have a grumpy emperor/senate remove them}
Blessing of Mars (lvl 30) - Gives a bonus to all combat rolls.
Blessing of Neptune (lvl 20) - Gives a bonus to all rolls related to the sea, from swimming, naval combat, fishing, etc.
I could do more, however, I think I've set the scene.