Rego Corpus Level 35 - Curse of the faerie jig
R: Sight, D: Conc, T: Group+3 size
Makes an army of up to 10000 men lose control of their feet, which start shuffling, hopping, tip-tapping and prancing in a mad, furious -- but well-coordinated, and not unpleasant to the eye -- dance. While under the effects of this spell it is impossible to move around under one's own volition, except perhaps by pulling oneself along a rope. This mad dance is exhausting: every minute of dancing requires a roll for exhaustion (as per corebook, p.178 - Stamina-Encumbrance roll of 6+ to avoid losing a Fatigue level).
Base 2, +3 Sight, +1 Conc, +2 Group, +3 size
No need to use a high-damage Ignem spell. Just use a low-damage one with a big area as well as a longish duration like Sun (or even Diameter). That will slow-cook them.
No need to get so complex. Pit of the gapping earth PeTe15. Increase the size to +4 and make it sight. Drop the army into a 9ft deep pit. Works wonders with horses. Change the same spell to MuTe to change the earth into air make it concentration ( long enough for someone to fall 9 feet) and release. Massive grave.
I think the PeTe spell is 40 and the MuTe is 45.
Eh. If its a slow cook, they might flee or break the ring.
Better to flash-fry.
Also, less chance of any survivors to report things to nasty Qaesitors, who are always poking and prying where their noses don't belong... grumble glumble grumble...
In the words of Flambeau the Founder: No witness, no crime; no body, no murder.
I think the way to make Ignem work is to up the duration on a lower damage spell so the wounds build up and eventually kill (sooner or later there will be a high roll on the stress die.)
Enormous Enduring Arc of Fiery Ribbons (CrIg 45)
R: Voice, D: Concentration, T: Group +3
As Arc of Fiery Ribbons (ArM5, p140) but the fire extends 300 paces from the caster and continues to do +10 damage each round that the caster stands in place and maintains the spell. If cast with a Rego requisite the caster may change the direction of the arc in the later rounds.
base 5, +2 voice, +1 concentration, +2 group, +3 size
Or use Multicasting spell mastery, with a score of, say, 4 x mastery (16 copies) on a spell that splashes an area in sight with fire damage (Modify Arc of Firey ribbons to splash out from a point within voice/sight range, as opposed to from your hand). Repeat for 5 rounds. Enjoy roast marshmellows with your friends.
Actually i think the average army would reject the liquor(if for nothing else, because they would be well aware of the danger of drinking too strong liquor), or possibly dilute it with water, because this was a normal practise anyway, as it was safer to drink water with some alcohol in it.
As there´s lots of room in it for change, i would add a magnitude more to make it smell and taste like water.
Then it should work.
A cosmetic effect does NOT cause loss of fatigue and wounds!!! This is not a cosmetic effect. And yes its the same as WoFS basics, BUT you´re also adding one more "full" effect, the fog.
Add PeIg(or PeCo if the effect is only against humans) as requisites and ill say its somewhere close to fine (of course, then there´s formally the problem that their base for causing fatigue is 4 and 10 respectively).
Or raise it 1-2 Magnitudes as you´re doing 2 primary effects, both at the Base 3 guideline.
Not sure how well it would work, but darn i would REALLY like to see it used!
Base 2 is only "loosing control". What you´re doing above is "control a target´s motions" which is Base 10 or MAYBE the Base 4 "control the largescale physical movements of a target", although im a bit suspicious of the exact meaning of that phrasing. Still use the latter and its still at the requsted level 45.
Say WHAT?
And BTW Noliar, why level 45 and not level 50? If you´re aiming to max out the formulaic spells within RAW limits, thats level 50 before you must go ritual.
So you can take the multicast mastery multiple times for the same spell? Does that work for other masteries like penetration and targeting? 
8)
No, the effect is really just loss of control of one's feet - the end result is the same as if they were spasming, which is canonical for Base 2. The faerie jig is just a cosmetic effect.
What I'm saying is that it's cosmetic the fact that it feels "supernaturally chilling" instead of "sulphurously toxic". In terms of damage it's exactly like Wreaths of Foul Smoke.
No, thick wreaths of sulphorous yellow smoke don't really allow you to see much further than a thick fog 
Let me repeat, this is exactly like Wreaths of Foul Smoke, with a different "look", and different Range/Size/Duration.
Creo(Rego) Ignem Level 35 - Summoning the thousand Hellhounds
R: Touch, D: Conc, T: Group + 2 Size
Conjures 1000 large hounds made of fire (each approximately the size of a large campfire) that rush to attack anything the caster commands them. A hound is as hard to smother as a large campfire ... that bounds and bites, causing +5 damage every round. A thousand of these terryfing monsters are enough to rout an army ten times as large.
Base for the Creo effect is 4 ("Create a Fire doing +5 damage" - the Hellhound shape is purely cosmetic)
Base for the Rego effect is 3 ("Move a fire quickly through space")
Final level 4, +1 Requisite, +1 Touch, +1 Conc, +2 Group, +2 Size
I think it does for penetration, I don't know if it does for targeting.
Remember, however, that mastery increases as an Ability, which hems in how many times you can take each. By level 7, its prohibitively expensive to add more levels.
You can pair this with Enchant Casting tools for Verditius to create some truly bad-ass Formulaic spells.
i.e. Demon's Eternal Oblivion level 5, Minor Focus in the material of the Casting tool, Perdo 10, Vim 14, Other lab total values totaling 40: Lab total 74. 74/2 -5 = 32. Enchant in one season
From Casting tool: +40 penetration (20 points), quiet, quiet, subtle (6 points), + 6 to casting total (just cuz)
Mastery: 75 xp (level 5): 5 x multicast - 25 copies. This spell now has +59 penetration, doing up to 125 Might-damage divided into groups of 5 however the magus chooses, cast silently and without movement.
Step aside, puny Flambeau Archmage--, this demonic horde looks like a job for .... duh duh duh duh Mid-Level Verditius!
I'm pretty sure this is not RAW. "Pilum of Fire Multiple Casting Mastery" is a single Ability, so you can't have that Ability more than once, any more than you can take Latin more than once. You already get multiple copies of the spell with a higher Ability score; that's the benefit.
I believe you future people call it oxygen. It's within the medieval experience that some airs lack the quality that makes them worth breathing (mines, smoke filled rooms, small sealed oubliettes, murder by smothering etc.). If yo destroy a quality with perdo it does not come back on its own.
45 makes it more challenging and makes the results available to younger magi - fewer hours of xp grinding before things go boom.
Its causing exhaustion, so no its not cosmetic.
Then you have misread WoFS, its smoke only blocks vision very close to where its created(5 pace radii). You´re creating "impenetrable fog" AND a WoFS in one.
No. Or rather, you can pick it any number of times you want for fun or whatever, but if you multicast the number of spells is still the same, 1+Mastery Score.
Once you stop destroying it, yes i would most definitely say that it "does come back on its own".
Otherwise you´re creating some very odd stuff. Literally, because you´re actually creating something unnatural by using destruction. Meaning it should be Muto territory.
Ok, i was just wondering if you were one of the many who had misread the RAW statement as being 50+ requiring Ritual rather than up to 50, including 50 doesnt require Ritual casting.
Im not sure how many times this has come up on the forum, but its many times at least.
Forest of Nightshade (MuHe)
R: Touch D:Sun T: individual +4 size
Turns all herbam material in a circle 10.000 paces around you poisonous. It causes a medium wound per Diameter where you are touching it. Dead horseys and dead/wounded soldiers easdily.
They most likely wouldn't drink the water at all - gives you the bloody flux dontcha know 
I think the easiest Terram effect is:
Creo Terram level 35 - The Big Clay Blob
R:Sight, D: Conc, T: Ind +6 size
Creates a huge blob of clay, about 4 paces thick at the centre and tapering to 2 close to the edges, over a circular area up to 2000 paces across, that engulfs and traps anything caught within it.
Base 1, +3 Sight, +1 Conc, +6 size
As for Herbam, the only Form left:
Creo Herbam level 40 - Rain of Javelins
R: Sight, D: Mom, T:Group + 5 Size
Creates a large circular cloud of (downward pointing) javelins of hard wood. The cloud is some 2000 paces across, a few dozen paces thick, and a few dozen paces over the target area. Each javelin is about half an inch thick, 5 paces long, and disappears upon striking. About 200 javelins fall every square pace - meaning at least a few dozen will strike any standing man, likely killing him unless under a thick, solid structure (a hand-held shield will likely be battered away by the first few hits).
Base Level 2, +3 Sight, +2 Group, +5 Size
Let me repeat it. I could have written the spell just saying that the feet of the victims violently spasm around. If a body part spasms for a sufficiently long time, you will be exhausted. Try sitting on a trotting horse for half an hour -- which is a much lighter exertion. The fact that the victims actually appear to dance is just cosmetic.
I think it's you who's misreading the spell. The smoke blocks vision only where it's created, which is a tiny patch 10 paces across. It's not clear to how far a distance within the patch it blocks vision, though it's certainly less than 10 paces (or you could just see through the whole cloud). From my experience with fog and smoke, I can say that thick smoke/fog does block your vision to a pace or so. I'd be willing to concede "a few paces" - not that it would change much the final result.