I was thinking of standardizing resistability to supernatural effects. None of the Forms or spell guidelines have any built-in mechanisms to avoid them functioning against you, and thus whether the canon spells can be resisted, as well as the EF to do so and what resisting them means, is pretty much decided ad-hoc for every canon spell. This makes it difficult to determine whether original spells should have the possibility of being resisted and what the EF to do so should be, and also means that the Magic Defenses available to hedge traditions are often essentially meaningless as they modify rolls you never get to make in the first place (see boosting rolls to resist Mentem effects, most of which don't offer rolls because screw your willpower). Plus, I personally think it makes engagements between muggles and supernatural beings more interesting (and non-magi characters in general more valuable) if the various instant win buttons of the game don't always go off to the full extent hoped for, and gives magi just that little bit more reason to enforce their "don't risk drawing the ire of mundanes" philosophy.
Anyway, for ease of use and consistency, I'll mostly crib the framework from D&D, hence the term saving throw. A successful saving throw only diminishes an effect rather than completely shutting it out in the vast majority of cases, to protect the vaunted status of true protections like proper Magic Resistance, Immunity Virtues, and True Love/Friend. (But some effects are simply all or nothing, so exceptions are inevitable.) Making a saving throw, like any other act of defending yourself, is generally an active process, so unattended objects, non-sentient plants, and willing or unconscious targets generally don't get them. My current idea for the save EF of an effect is to make it (Technique+Form+base effect level)/5 or (Might+base effect level)/5, but I think this may need to be squished so that lower level effects are a bit harder to resist and higher level effects are a little easier; let me know what you think. The resisting roll is just a stress roll of whatever Characteristic and Ability is circumstantially appropriate.
Due to the breadth of effects possible to design under the Ars rules, you'll have to decide what a spell being saved against means while designing it, but as a guideline it should halve the intended impact of the spell, either by weakening the effect itself or making it less permanent, in a way that substantially benefits the defender. For direct offensive spells, this might mean halving incoming damage from a CrIg fireball or reducing the severity of a PeCo wound, while other negative effects might be less effective at what they meant to do or have results less permanent than intended; for Mentem examples, a resisted sleep-inducing effect might make the character woozy with drowsiness for a penalty but still conscious, while a resisted memory-destroying effect might leave the memory partially intact with the details blurred out, or make them temporarily forget but able to restore the memory with sufficient association and reminders.
Let me know what you guys think! All forms of critique, suggestion, and bitter flaming for how stupid and fun-hating I must be are welcome.
I'm not suggesting this replace Magic Resistance; my apologies if it came off that way. The ability to completely and reliably shut out entire supernatural effects is bonkers powerful and should be a defining characteristic of the few that have it, as it is in standard Ars. This is more about shifting the balance of power across the entire setting by making competent, lucky, or numerous mundane characters more able to survive against the supernatural, albeit not unscathed. I think this could make the game more exciting, both for the tension it adds for magi characters, and for companion and grog characters it makes going up against supernatural entities less of a "fuck you, stop having fun, I'm eating your character sheet and you're not allowed to do anything about it" experience. Plus from a setting perspective it justifies more why magi have a mundane non-interference clause, because now a king and his armies aren't quite just going to evaporate into mist all at once the first time a Flambeau looks in their general direction. In canon it's just sort of "idk, it might piss God off? Better not chance it."
The term may be the same, but the mechanics behind the rule have changed several times this century. In the 20th century you had the classic five based on situations. For some reason these were changed around the turn of the century, no idea how those worked, and they changed it again in the current version.
I am presuming that is what you mean, as the modern version of the classic game was redesigned to appear more like Ars Magica. It now uses saves based on Ability scores, which makes no sense to me but let's just go with that. Now, in Ars Magica, back in 4t edition (where I started), there used to be "Natural Resistance" rolls. The spell description would define what characteristic to use, but you would roll that plus a stress die for an Ease Factor of 6 or 9 (depending on the spell, and I think those were the only two benchmark numbers).
It's a nice, simple, idea - I like that. I think it would indeed work well to make magic more unpredictable and overall increase the power of mundanes (and really any other opponent).
Note that it greatly increases rolling in game. If you now throw an aimed spell, you need to roll concentration, to roll the spell casting roll, to roll the aiming roll, perhaps a damage roll, and now - also a saving throw. If you throw a super-sized fireball against 12 opponents, you now need to roll 12 saving throws.
Incidentally, I personally prefer to make mundanes more powerful - among other benefits - by doubling all magic difficulties. But that's not really related much to your house rule.
I'm not really a fan of the saving throw idea, but maybe that's just because I'm opposed to seeing ArM come to somehow more greatly resemble any edition of D&D. I also still use 4th ed., so...
I feel the basic fact that "magic goes last" is the greatest "equalizer" mundanes can get against magic (especially in an alternate setting with things like shotguns, assault rifles, and claymore mines).
Did 5th edition remove that particular rule? I understand it's traditional to completely re-write the combat system from edition to edition so I wouldn't be surprised if this had gone away.
That's a good point, YR7! Though, actually, I kinda figure that if an aimed spell already requires a roll to hit, then your attempt to dodge it "counts" as your saving roll for that case - the mechanic is already built in, as such. But yes, it surely does increase rolling in the game regardless, by giving a defense roll against actions that were normally automatically successful. I wonder, for the case of combat groups, whether it would be better to have them roll a group save rather than individually, same as when they're defending against physical attacks; that might at least soften the impact in the situations most likely to be impacted by stopping to roll a bunch. Thanks for the input!