Are there any guidelines as to the nature of fast-cast spontaneous spells needed to counter a magi's spells?

Question

In Ars Magica, a magi may counter incoming spells and effects:

In order to create a fast-cast defense against magic, a maga must know the Hermetic Form that governs that magic. If the caster is a Hermetic maga and is using words and gestures that she can hear and see, she may determine this automatically. ... In general, a fast-cast defense with half the level of the attacking spell is enough to protect the maga herself or one other individ- ual. The spell still takes effect, and the side effects of deflecting the spell may have to be dealt with. A fast-cast defense that matches or exceeds the level of the attacking spell can completely neutralize it, probably in a fairly spectacular fashion.

Are there any guidelines as to what, exactly, constitutes a counter?

Specifically, I am building a Villain (effectively Dr. No) who is built around countering incoming spells. He plans to use the fast-cast spell mastery:

The maga may Fast Cast the mastered spell according to the rules for Fast Casting Spontaneous spells, on page 83. The maga still takes the –10 penalty to Casting Score and risks the extra botch dice, although the botch dice will be partially offset by her Mastery Ability.

Along with a number of formulaic spells to shut down incoming magic.

What set of formulaic spells does he need in order to deal with almost all incoming magic? In a more general sense, are there guidelines for appropriate spontaneous spells?

Answer 1

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. 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:

The base answer is that the rules allow for a Vim spell to give protection, and a Form specific spell if the inbound Form is known. I dislike the Vim cover all solution, but it is in the rules (somewhere).

• Fast casting applies 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).
• therefore I extrapolate 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.
• 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. I'd say that any Rego Vim or Perdo Vim effect must equal the attacking spell level (without considering Penetration).

edit to add ref: ArM 5e, pg. 161. Rego Vim Guidelines: Sustain or suppress a spell cast by another with level less than half the (level + 5 magnitudes) of the Vim spell.