Here’s a thing that’s been tossed around my troupe for a while now. One of the least appreciated facts of the spell creation guidelines, we think, is that Group is a set of up to 10 individuals and this multiplies by 10 for each additional magnitude. A few of these have been the stock in trade for our examples of how to break AM for ages, some are more recently as it’s been coming up a fair bit in conversation lately, and we’d thought sharing might be fun. These spells showcase the... unique results... of taking exponential scaling in the spell creation rules to their logical conclusion, and in doing so completely violate the central rule of spell creation!
Of course, there are some ground rules. A spell that breaks a system as cool as Ars Magica’s must do it with style.
1) They must have a rather silly result. While destroying a city in a rain of fire or flooding a valley is impressive, it feels cool and plausibly medieval for a powerful magical effect to do that. These spells should generally show how odd the scaling is with utterly inappropriate effects such as, I don’t know, constructing a sand castle the size of France.
2) They should usually emphasize the potentially underappreciated fact about Ars Magica that pretty much any focused specialist can end the world or utterly change it in a few spells if they try really hard at their chosen field. Even melon-farming.
3) They should not be rituals, if at all possible, and so should be exactly level 45 or less (ideally 30 to make them easily used with minimal fatigue even with a starting character who isn't optimised much at all). Formulaic spells utterly changing the face of history and nations once per combat round is more impressive than a ritual for demonstrating this effect. Some amusing rituals may be included that are levels 80-100 simply because they are great, and quite doable for a focused elder magus, even if they cost vis and take marginally longer.
4) Whether explicitly discussed or not, unless completely impossible there should be a simple and obvious solution to the mechanical problem, such as reducing or removing exponential scaling of targets or modifying how base targets are reckoned. It's not nearly as fun or wacky to make spells that highlight major, insoluble flaws.
The three key features of these spells currently being displayed are:
1) Exponential scaling. If you can do something at base level 5-15, you can do it 100k to 1 million times more at levels 45, which isn't even that hard in the grand scheme of things.
2) Silly base targets. Several animal and herbam spells imply you create some things by weight, while giving absolutely generous max individual targets as if you were intended to create one individual thing. (Creo-ing things by weight rather than ideal unit also isn’t very Platonic and should be avoided if possible, although it's usually pretty unavoidable with undifferentiated continuous base matter like Terram or Aquam.)
3) Part Target + Size to affect continuous material forms such as Terram or Aquam in 90% of situations outclasses Boundary Target + Size, and can be arbitrarily shaped without being a viable target for any other category. While it's rather minor, without this dodge, it would be significantly harder to affect all of europe rather than merely a duchy or two with the upper levels of hermetic magic. (Removing this dodge would make it nearly impossible to affect seas or oceans with reasonable aquam spells, however, and it's not at all a simple issue to fix)
The fun part: Quite literally every single one of these effects was discovered accidentally by the authors while messing around with spell systems and realising that a level 20 spell with a cute group target effect could be bumped up to a 45 and have 100,000x the cute effects. In fact, the story of John Melitturgus of Jerbiton mostly actually happened (but the spell was never cast in a game- it’s just that the mistake on how targets/sizes work did happen). (Although the fearsome magic of the Societas Peponis sadly is just an in-joke, such spells don't exist in-saga.)
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Produco Nimbum Magnum Apium (A Spell for Summoning a Significant Number of Bees)
Creo Animal atque Rego eadem, level 45 [magnitude 9]
Range: Touch
Duration: Diameter
Target: Group (Size +3)
This spell was created by Iohannes filius Porphyrii Ierbitonis Melitturgus, a hermetic bee-keeper (stranger things have happened) who took a short detour in his middle years from doing Rego waxwork to learn some self-defence with the Ars Creandi. Since he was already skilled in Animal magic and bees in particular, he decided to create a powerful spell to summon a large swarm of bees to defend himself, with terrifying consequences. Iohannes figured 10,000 bees would be enough- a small hive’s worth.
However, this spell turns out to create an utterly infeasible amount of bees: it creates 10,000 base targets. As a welsh pony weighs about 500 pounds (the animal base guideline) or pig weighs about 200 pounds (some p.117 CrAn spell guidelines imply this is the base) and a bee weighs approximately 10 ounces per 1,000 individuals, this spell creates approximately 50 billion bees which erupt out of the entire exposed area of the body of the caster in waves, creating a cloud of solid bee almost precisely 20 paces in radius, which will rapidly spread out subsequently.
The bees are under the mental control of the caster, who can upon creation direct them to do whatever sort of thing comes to them when 50 billion bees are available. Bees given an order cannot be given new orders without a Rego Animal spell cast at them as a +1 Requisite is probably insufficient to fully mentally control them. Hermetic magic is frequently fire-and-forget, so decide carefully. Bees created by this spell vanish in 2 minutes, so you can have out at maximum 19 times this number of bees if you cast constantly without becoming fatigued, or a swarm of nearly 1 trillion bees. This swarm should generally be capable of reaching anywhere on a battlefield and able to fly over castle walls and into towns within this time.
In addition to being an utterly terrifying and silly spell, this spell would likely cause and/or imply a mathematical revolution in the order of hermes in order to create numbers to describe its effect in the lab text, or else how was its effect composed?
If you are going for sustained swarm numbers, rather than immediate response to something, another level 45 version with Diameter: Moon, Target: Group (Size +1) will summon only 50 million bees but for an enduring period. This produces up to 150 billion bees per hour of work. A dedicated Apiomancer could summon 6 trillion bees in a 40 hour work-week, which could literally blanket entire nations and would probably be noticeable by everyone in an entire tribunal in practice since they will not stand end-to-end in a carpet but will spread out to do the Apiomancer's bidding (although what precisely that is is admittedly unclear).
A level 60 ritual version with locusts could be quite nasty, capable of exterminating all human life in mythic europe due to cancelling the next harvest and eating all vegetation continent-wide. But that wouldn't be nice.
Another level 60 ritual version would be permanent and thus gain +4 size magnitudes (+1 from momentary, +3 from increased level). Two of these will be spent on 2 more magnitudes of Rego Requisite (now 15 entire levels of Rego requisite based magnitude, similar to Rego Animal 15 Utterly Control An Animal). This should theoretically give 1 trillion magical bees permanently under the caster's absolute mental control at the cost of some vis.
Note that botch reduction is vital with this spell so as not to be surrounded by billions of magically enraged bees. However, your powerful grasp of animal should grant you at least +5 form bonus soak so it may be impossible for a bee to sting you as your skin is as resistant as leather scale armour and your bees may not be able to penetrate your own parma.
(Creo Animal 5 (Create Insect [1 Base Target of Bees]) + 1 Touch Range + 1 Diameter Duration + 2 Group Target + 3 Size [10,000 targets] + 1 Rego Requisite))
Nota Bene: As odd as it is, base targets for vermin in the corebook clearly work off of mass in Animal, as the weaver's trap of webs and curse of the ravenous swarm spells show. I personally believe summoning vermin should summon them by the swarm or hive, or set of webs for products, not the ponyweight.
QUO... SUNT.... OMNES? (A Spell Which Will Find Persons)
Intellego Corpus, level 80 (magnitude 16) [RITUAL]
Range: Touch
Duration: Momentary
Target: Boundary (Size: +9)
[Yes, these are supposed to not be rituals, but this was just too much fun. It’s also purposefully overengineered and impractical.]
This spell senses all pertinent information about all human beings on one continent (about the area of the Roman Empire- +1 more magnitude will affect all of Mythic Europe using 'the Known World- the Sahara, the Urals and the Indus' or something similar as a boundary) as long as you can find a boundary relevant- the shorelines are a pretty good bet for part of the boundary, or 'the entire Mediterranean' to find someone at sea. You can learn the exact height, weight, sex, humoural balance, skin colour, eye colour, hair colour, health status, and more with this spell. As long as you know who you're looking for, you can find anyone's location, as well as all of their utterly identical twins, as long as you know enough about them. Casting this spell forcelessly will also reveal to you all Magic Resistance in Europe by the blank spots in your information. And at the same time it will prevent you from violating the Code of Hermes harder than was thought possible as you avoid scrying on every single one of your sodales in one go.
It's a lot of vis, but there are some theoretical uses. Maybe you have to find out where a person is in all of the Muslim world, without an Arcane Connection. For 16 Corpus Vis, it's not too unreasonable. Failing to find them proves they have magical protection, but probably all of their known associates don't so you should still know roughly where they are. It could also be useful for demographic investigations of the mundanes, should the King of France want a census done and is willing to pay 50 Vis worth of magical resources or something for your trouble (maybe rights to a royal forest full of magic for a generation or something). Using it on commission is hardly the least of your worries about the Code of Hermes with this spell.
(Intellego Corpus 10 (Know All Pertinent Facts About Someone's Body) + 1 Touch Range + 4 Boundary Target + 9 Size)
Mismaking the Map for the Territory
Creo Animal atque Aquam, level 100 (Magnitude 20) [RITUAL]
Range: Touch
Duration: Momentary
Size: Group (Size: +14)
This spell creates a 1 to 1 scale map of mythic europe, coating the entirety of your campaign setting in sewn-together sheets of parchment. The detail is down to the character’s knowledge of maps, so it may not reflect cartographic reality without appropriate finesse and philosophiae (geography) checks unless Intellego and Terram requisites are included to have the spell detect what it will be overlaying and have the ink display that (probably requiring another +2 magnitudes for requisites as well).
(Creo Animal 15 (Create Treated Animal Product) + 1 Touch + 2 Group + 14 Size + 0 Necessary Requisite (Aquam for Ink))
Grandinat Melopeponibus (A Societas Peponis Battlefield Weapon)
Creo Herbam, level 30 [magnitude 6]
Range: Sight
Duration: Concentration
Target: Group (Size +3)
The Societas Peponis, an Ex Miscellanea group of herbalists who grow magical melons, was tired of being made fun of. They had seen that they were the go-to example of "Not all magical skills are equally valuable — if you waste all your virtues and training learning to be a hedge wizard Ex Miscellanea melon farmer, you won't be as good as other mages." Infuriated, the members of the Societas Peponis created this series of spells to prove they were at least as deadly as a Perdo Corpus or Creo Ignem specialist, if not even more so. This spell was created to destroy entire massed formations of troops threatening the melon fields of the Melon Lord of the covenant Peponica Magna.
This spell creates 10,000 melons one pace in diameter (base target size) in some kind of group formation over an area, usually a 100 by 100 square with each separated by an arm’s breadth, 4 storeys up. This is dropping projectiles equivalent in weight to perhaps 800 elephants or well over a thousand of cow, which will probably bludgeon any formation of men underneath to death. Damage is probably about +15. Unfortunately, the spell must realistically be sight range as you don't want to drop 4000 tons of projectiles packed within your own voice’s range.
Concentration is held until impact, then the projectiles are dissipated with losing concentration, leaving a completely empty battlefield besides the shattered corpses of the enemy. This spell can also utterly destroy an average village, punching through peasants' homes and inflicting massive casualties.
(Creo Herbam 1 (Create Plant [1 Base Target of Melon]) + 3 Sight Range + 1 Concentration Duration + 2 Group Target + 3 Size)
Notes: This spell probably is already dubious, but it’s well worth thinking about why. Example Creo Herbam spells imply you must create plants into a sensible location for them, or at least into your hand- you shouldn't create them in midair. This would also prevent Creo Animal elephant bombs.
However! You can still do these, presumably, (based on MuTe[Re]10 the Crystal Dart) with +1 magnitude as a Creo Rego-que spell to create on the ground and then fling the created object, though no spell guidelines exist for animal or herbam movement in that sense (corpus has some guidelines for levitation, so it stands to reason using Rego to levitate plants and animals should also be possible). They would have had to be aimed anyway if you're creating them hovering over the target, so it's not a big loss.
One may also argue it cannot create melons that are 1 pace in diameter naturally, in which case it should instead create however many normal melons are needed. From a height they'd still be deadly. Or it could use extra magnitudes for 'unnatural size', like how unnatural properties take extra in aquam or auram spells. There are similarly no guidelines for making a winged rabbit or an antlered dog or other unnatural animals compared to a normal hawk or cow, nor guidelines for unnatural plants, but it sounds reasonable. I'd generally say unnatural Creo spells require at least one extra magnitude, but thousands of cows worth of hard-exterior projectiles filled with water (a dense substance) is a pretty big error margin on deadliness.
Incantatio Quassanda Castrum Saxifrago Melopepone Magno Ultima Magnissima Trismegisti (A Societas Peponis Siege Weapon)
Creo Herbam, level 45 [magnitude 9]
Range: Sight
Duration: Concentration
Target: Individual (Size +8)
Of course, they couldn't just stop there. In cases where the Societas Peponis has to go on the offensive, they need a spell that can destroy an entire order of the Knights Hospitaller and the greatest fortress in Mythic Europe in one formulaic spell, since there might be something else worth 10 seconds of time to do that day. This spell was inspired by viewing the great castle Crac de l'Ospital (the future “Krak des Chevaliers”) and the pyramids of Egypt during the Societas's research trip on exotic desert melons.
Oh, and they wanted to create a spell to do this that their novices could do if focused and specialised in their preferred arts right out of gauntlet. It does cost them fatigue, and they can't do penetration with it well until they're a bit older.
This spell creates a single melon with the volume of 100 million base targets in the sky above a target of the magus's choice, within sight range. The one-third a mile wide melon looms above like a second moon descended to Earth, then, surprisingly fast for its bulk, slams into the target. The melon is approximately the mass of the entire great pyramid complex at Giza, and outweighs any castle by a factor of 200. It's not quite the equivalent in force to picking up the entire Rhine and throwing it at someone (that would take a few more magnitudes to get to and hence be a reasonably complex ritual), but it's close enough.
At its mass, the fact that it's a melon and not the world's closest re-enactment of noah's flood since the event itself is immaterial. The tidal wave of melon juice and pulverised bodies and stones would settle to become a new lake, or a flood down from the motte of a castle to sweep over the surrounding villages... but inevitably the scale of destruction will surprise the caster enough to lose concentration.
A pit to take cover in and a ward against stone is heavily recommended as the shattered shards of the city it is used on will not vanish upon concentration-loss. Even at 50-100 miles per hour the melon will produce a rain of stone fragments across a surprising area.
Forceless casting of this spell at one another is great fun for magi of all ages.
(Creo Herbam 1 (Create Plant [1 Base Target sized Melon]) + 3 Sight + 1 Concentration Duration + 8 Size)
Notes: If creating an unnaturally sized plant requires extra magnitudes (nothing seems to say this outright, but 'wood in an unnatural shape' requires +1, so perhaps +1), this would reduce the size, but even +3 (or +2 for very unnatural, +1 for rego requisite) would still make it hitting a castle with another castle made out of water(melon).
Incantatio quae Stagnum Siccaret (A Spell For Draining A Swamp)
Perdo Aquam level 35 [magnitude 7] (optionally, 45 [9])
Range: Touch
Duration: Sun
Target: Part (Size: Normal. Optionally, +3)
This spell creates a zone of destruction in a liquid 15 feet across and 6 feet deep, destroying all water that flows into this region for the remainder of the day or night. For best practice, swim down a bit so that it does not only skim the top level off, especially if the body is small but deep. However, it does not destroy the salt in the water, so that it can be harvested afterwards if the body of water itself is depleted. This spell has a number of uses:
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If cast in a river less than this size, it will instantly dam it and destroy all water that enters as fast as it flows in. This will cause the river to dry up as fast as the last drops can flow downstream, although the bottom will remain muddy and wet with sporadic pools.
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If cast in a small pond or swamp, it will reclaim land, although not instantly. It will take a dozen or more castings to reclaim a normal lake of about 300 acres in size. A larger one like Lake Geneva is effectively with this version of the spell impossible, taking over a million castings.
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Normal salt water lakes will produce about 90-120 tons of salt per casting, although only upon the final few castings will the salt be readily available (a little bit of extra magic will be necessary to remove remaining pools that didn't flow into the perdo zone unless the lake bottom is perfectly slanted towards the perdo effect and the effect is placed on the very bottom). Remember that once volume is reduced by about 7x, life will no longer be able to survive in the body of water.
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If cast on the ocean, nothing notable happens. Unless...
Magi should be cautioned it is far easier to incautiously destroy a lake than create a new one- the vis costs of creating a lake are quite large, and it would be easier to build a channel to the ocean and then optionally use a variant of this spell (destroys Salt in Salt Water but not the Water) to clean it back up if it was fresh water before.
(Perdo Aquam 15 (Destroy Part of Liquid [Water, but not Salt]) + 1 Touch Range + 2 Sun Duration + 1 Part Target)
There is a variant level 45 spell, Incantatio ut Lacum Magnopere Combiberet that sacrifices the ability to preserve the salt for the ability to drain any lake in europe within half a decade. This is proof that order of magnitude scaling for targets is, as the magi would say, valde insanissimus.
(Perdo Aquam 10 (Destroy Liquid [Water]) + 1 Touch Range + 2 Sun Duration + 1 Part Target + 3 Size)
Incantatio quae Novaterram Batavianorum Condet (A Spell For Creating A New Nation)
Creo Terram level 40 (magnitude 8 ) [RITUAL]
Range: Touch
Duration: Momentary
Target: Individual (Size +8)
This spell creates 100,000,000 cubic paces of mixed stone and clay in the form of a wall 8 paces wide, 20 paces tall, and 355 miles long, enclosing an area. While the uses in fortification of course are obvious, if sunken into the ocean at an appropriate distance from the shore (where the depth to bottom becomes approximately 12-18 paces) it will wall off an area no smaller than Wales which can then be reclaimed using water draining or the next entry, Cors Novaterrae Batavianorum)
The walls may need to be lowered for convenience on the shoreward side. Doors through your new country's wall, will require a Finesse roll. Craft checks for dykework or masonry are largely unnecessary given the sheer volume of land being summoned.
New colonists to your nation will be quite happy with its wealth of land, once plants and animals migrate in, but the previous fishermen and coastal lords may be very unhappy. Therefore, cast twice to make a fortification against them.
(Creo Terram 3 (Create Stone) + 1 Touch Range + 8 Size)
Item: Cors Novaterrae Batavianorum
Effect One: Convert One Wales Of Shallow Water Into Land, Muto Aquam ad Terram 84 (magnitude 16)
(Muto Aquam [ad Terram] 4 (Change Water to Earth) + 1 Touch Range + 2 Sun Duration + 1 Part Target + 12 Size + 1 lvl 2 uses/day + 3 lvls environmental trigger)
Shape and Material: Aquamarine gem (+3 material bonus to water) set into a Silver Chalice (Small Silver Object) that is filled with Mercury (+5 material bonus to muto)
Item Type: Invested Item (12 vis to open, min 6 Magic Theory). 8 vis to invest the power into (modified level of 76-78 depending on magic theory).
This chalice should be carefully placed on the surface of the heart of your new walled off ocean (you should be in a very stable boat or water-walking) and told the command phrase- Fiat Humus. It will convert all the water it touches- the entire Lake Novaterra - into earth. The size is such that it will always succeed if all the water is connected to all the rest (it should be, being as it was the ocean until recently).
Silver is used as gold is unnecessary and it's amusing that it's a metal that is famous for tarnishing that itself can never tarnish because it converts all water to dirt.
You should probably guard this item carefully as if damaged your entire realm instantly sinks beneath the waves when the effect fails. Maybe conventional water draining or filling it with dirt is more safe...
Lead Weight of Catastrophe
Effect One: Literally Destroying This Entire Ocean, Perdo Aquam 49 [magnitude 9]
(Perdo Aquam 10 (Destroy Liquid) + 1 Touch Range + 2 Sun Duration + 1 Part Target + 3 Size + 1 lvl 2 uses/day + 3 lvls environmental trigger)
Shape and Material: Small lead weight (Tiny Base Metal object) inscribed with runes of caution and disaster. No bonuses, but sufficiently ominous and durable.
Item Type: Lesser Enchanted Item if you can (PeAq lab total 98), otherwise Invested Item (5 vis to open, min 3 Magic Theory). 5 more vis to invest the power into.
This item is a small lead weight, when a command phrase is spoken ("Misereatur Deus"), activates and will thereafter continually function. When thrown into the deeps of the ocean, it sinks down and destroys the world. Slowly.
It will continually, each day, at sunrise and sunset, renew its effect of destroying an area of water each round (at bare minimum, perhaps even faster). Forever.
However, fear not. It would take thousands of generations to fully drain Oceanus. Even the Mediterranean, which would become a vast, deep salt flat, would take 110 generations. It is likely that the end of the world will happen before then, so this isn't so worrisome. More important would be the fact that the sea would recede from the shores by a hundred yards within the first few generations, and that the Pillars of Hercules, Red Sea, English Channel, and Skagerrak amongst others would close by then. If incautiously thrown into the shallows rather than the deeps, the effect will wear off once it is on dry land, assuming you do not walk to the item, sail out into the new borders of the world-ocean, and throw it right back in.
It is not likely this item would work due to the Limit of the Divine, not for any theoretical reasons but for actual practical ones. Since it is easily capable of destroying the world, one must imagine that the magus will be visited by angels warning of dire consequences if they continue in their work. It is likely, should the magus persist, the Most High will personally intervene by simply silently suppressing the item when it thrown into the ocean, rendering it a perfectly useless lead weight. The character may or may not keep their Gift, depending upon if God's Perfect Justice and Divine Wrath would demand that punishment (... probably).
Of course, no one has ever tried it. At least, no one has apparently ever thrown one into anything bigger than a small pond.
Post Scriptum: A variant that would merely de-salinify the world-ocean would take 10 times as long or else be 5 levels higher. Further every 5 levels added to the enchantment should also make it be 10x quicker due to increasing the size of the perdo zone, so if Hermetics can achieve a level 99 spell in an item, which they can in canon, they should be able to destroy the ocean in 6 minutes. Flow rates will probably prove to be a limit well before that level, but it can be accelerated at least by several orders of magnitude. Since you won't need 70 years (or even 7), you could probably make this item cheaper as a lesser enchanted item since it won't need to last long.
What can we learn from these items? Clearly, although amusing, these effects shouldn't happen. The main source of amusement in all of these is order of magnitude scaling- mages can cast spells from about levels 4 to 45 for non-rituals and 20 to 100, and targets scale massively over that range if the effect is weak and doesn't need lots of penetration to defeat magic resistance.
In practice, the abuse of group targets isn't a serious problem for the game, since if your players try to make something like this you generally just give them the look. In the case of John the Beekeeper, everyone had a great laugh when the actual size of the effect was realised and the problem was fixed by handwaving that the Platonic Ideals of Animal says the base target for vermin is one hive/swarm rather than the millions you'd get by ponyweights. But what if you did want to fix these in a hypothetical Ars Magica 5.5?
Well, one could prefer scaling by doubling, rather than order of magnitude. Group would still be 10 individuals, but each size would add only double (so it would take approximately +3 magnitudes to equal one old Size magnitude). This would allow low level effects to hit millions of targets in rituals, and pretty impressive effects to affect entire cities at the upper level of rituals. But the silly effects above that depend upon stacking 8+ sizes would find themselves affecting only football pitches rather than the entire campaign setting unless they can come up with two dozen extra magnitudes. On the downside, it would make affecting a crowd of 100 targets take +4 sizes instead of +1, and 1000 targets take +7 instead of +2, which is could be punishing to some legitimate applications of spells. It's hard to make a system that exponentially scales allow both the magic you want and disallow the magic you don't want- you want mages to be able to create New Batavia, but not trivially, and you don't want being able to light one man on fire at gauntlet translating into lighting nations on fire as an elder magus.
Ultimately, the only truly worrisome thing on this list is the Perdo Aquam effect. It was initially designed for a legitimate usage- reclaiming land for a Covenant using repeated Perdo Aquam casts. Then it was realised that durations can be used with Perdo according to ArM5 p.112 to create a continual destruction effect, which makes it a lot easier to drain a lake instead of just a small pond. Then it was realised that it could be enchanted into an item perpetually and simply thrown into the depths, since the water would naturally flow into the zone of destruction. Then "Wait, what if you threw it in the ocean?" and the room dissolved into laughter. An apocalyptic weapon had just been brainstormed by accident. Perpetual Perdo items are a truly scary thing and one must imagine there's rulings against them in the code.
It's amazing Mythic Europe hasn't been blanketed in bees or parchment or the red sea temporarily dried up by a curious magus looking for the pharaoh's chariots. Most magi must be saner than players. 
It´s quite funny how many do this mistake actually. Because you´re far from alone with it. While this was one of those things i read once first time i read the rules and just accepted it correctly from the start(which is definitely not true about all rules!).