There was another Ars Magica 5 question on Stack exchange, and like a good little opinionated community member I wrote a brief answer. I thought I’d share it here because it is an area that is a little unclear to me.
I wrote the answer below, which is also tweaked a little for clarity:
There are many layers to the idea of using fast-cast spells in ArM5 and the rules are unclear. This has led to many discussions on the official forums as groups seek to clarify or expand what is possible and logical. I didn’t like the outcomes of that discussion so didn’t commit it to memory. Instead here is how I thought the rules should be used:
- Fast casting can apply to both Formulaic spells and Spontaneous spells.
- The defensive spell must be half the level of the attacking spell to defend against it.
- The caster of the defensive spell must know the Form and Tech of the attacking spell to be able to defend. This implies that the defender must know that the spell is being cast (somehow).
- I think there is a Perception + Magic Theory to be able to identify the attacking spell’s Form.
- Therefore I surmise that once the attacking spell hits the defending Magus it is too late to fast-cast Spont a defense; as the Magus only has their Parma to defend with.
- Therefore too a Defender who is unaware of an inbound attacking spell cannot fast-cast against it. This makes Silent Magic and Subtle Magic virtues much more powerful. Your villain might be able to cast without words or gestures, and therefore nobody can really fast cast defend against the spells.
- Both Spont and Form spell applies the -10 cast penalty, and the botch dice. This makes a fast cast Spont spell very hard to cast. Which is in turn why many magi choose to lean Form and tech appropriate generic Formulaic spells which they then fast cast.
- Our troupe decided that the Technique can be any which seems appropriate to the attacking spell. So the style of the defending spell could be Rego or Perdo for example, as both easily can move or eliminate the Form. It could even be Creo or Muto too. I am not sure how Intellego would be used to defend against an inbound spell. Very circumstantial.
- That means that it is plausible that there is a general spell for defense in each form. e.g. “Defend against the Marauding Fire”, as a PeIg (gen) spell. So the Magus learns that spell at a level they like, and then hopes that they can fast cast it with the -10 penalty to the roll. As their skill increases they might learn a few versions of that spell.
I didn’t like the idea that a Defending spell could be learnt as a Rego Vim or Perdo Vim variation and still need only half the level of the attacking spell to neutralise it, but I do agree that the same Perdo Ignem spell above could be altered to be Perdo then all the forms to get the same style of effect. That would be a hard spell to cast given how the 9 additional Form requisites would affect spell casting totals.
I’d say that any Rego Vim or Perdo Vim effect must equal the attacking spell level (without considering Penetration).
But now I realise that there is much more to this than the question. There is a sub-set of questions around how Finesse and Quickness stat might apply to fast cast spells, and how a spell might be fast cast vs a weapon swing.
I cannot answer those as they are too deep into the RAW and frankly I’ve not the books here, and I’m not sure I could write it out without supplying too mach material that is copyright of Atlas Games.
Here are some further things to ponder:
- I think Formulaic spells should be “after” most other combat actions. I think this is the default stance in the rules, given the time it is implied they take to cast.
- I think that the rules allow for several fast cast spell to be cast in the same round, and that any fast casting performed means you cannot cast normally (spont or form) in the same round.
- I think Spont Casts should use a speed rating to determine initiative order. This gives us Form spell outside normal initiative order, and ordered by the caster’s Quick+Fin, but slower than all other stuff.
- There is probably also a place for using fast cast Formulaic spells with the multi-casting abilities too, but that is way too complex for here. A higly focused combat Mage might use a low-ish level Mastered spell which is Multi-cast and fast-cast to pepper an opponent with spells. It is exactly why a normal person should be afraid of a spell caster.
- Consider that a Formulaic or Spont spell probably going to be slower than a melee action or ranges weapon attack, so for a cast to stop a weapon swing they should use a completing Quickness + Finesse roll to determine their speed. This is where fast-casting come in, to allow the caster to always act “before” the swing.
- A fast cast is plausible with a short 0-2 second period where you can react instinctively, but no prepare at all. Any slower than this and it is probably just a spont or formulaic spell. This is how it feels to me as written.
- I dislike the idea that fast cast can be used for a range of actions, to interrupt each one. There is a rule in the core for making additional fast cast actions in a single round, but I’ve always discouraged their use.
- I saw an example between three casters, A, B, and C; where A cast a formulaic spell at B, B defended with a fast cast spont spell, but then C tried to neutralise the spell that B was casting. It gets messy and should come down to the relative speed of the combatants.
- I like using combat speed as a measure as it links well with weapon speed for non-magical combat and it adds value to the Virtues and Flaws which deal with spell casting speed. I pity the Magus with the Slow-Casting flaw who gets into a spell fight.
- There should also be a Concentration cap on the amount of things a caster can try. Each successive action should apply a further negative to the Concentration rolls, and before each action the caster is required to pass a concentration check. Failure means they’ve reached the maximum number of things they can fast cast in a round.
When I run my next Ars Magica game I’m going to strip this entire system back to its roots and rewrite it, but for now I hope this helps a bit.