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).
In Ars Magica there is a tendency (for me at least) to try to create spell effects and items that are powerful and flexible. When doing this it tends to make the final artifacts expensive in terms of time and resources.
I was thinking about a second approach, which would exploit the rules for quasi-permanent effects which the Duration: Ring provides. Essentially Ring is a duration that lasts for as long as the ring is maintained. With that in mind here is a few sneaky effects.
The Persistent Torch
To start with get a craftsman to create a sturdy circular compartment, something solid which the grog can drop and not break, anything really as long as it has a lid that will cover the insides well. In the design you need to note a few aspects: a place for the ring to be either carved or attached on the inside, and a separate space for the item which is to be enchanted. In layman’s terms this could be a very small wooden box with a clasp which shutters the insides.
The craftsman insets the separate piece of some material that is inexpensive and easy to enchant in the middle of the box – I’d choose wood as it will be darn cheap. Avoid metals and stones as affecting them with magic is harder than wood, and there is no need to blow out the spell size.
Then the effect is simple – a Creo Ignem effect at level 10 to 15, with base effect level of either 3 or 4, +1 mag for Touch, +2 for Ring. It really depends on how powerful the light is meant to be. A base 3 light is a torch equivalent, and this seems useful for a grog wishing to travel without attracting too much attention – at as a level 10 effect could be spont’d easily.
A base 4 the light created is as bright as a cloudy day, which is more than enough, although spont’ing that level of effect will take some effort for some younger magi. Either way it is castable, and creates a portable light which can be given to grogs; or perhaps even given to mundanes as a gift. Light has a huge quality of life benefit, especially to the poor.
As alternates duration Ring can be used for these other purposes too:
Granting heat and warmth to devices which increases the comfort of an inhabitant. Perhaps a carpet or small bed could be cast upon after being inscribed with a ring. Creo Ignem again for the effect, approx level 2 base effect will see a final effect level 5.
Create a fireplace which never expires. CrIg around level 5 as a base, with 20 as the final effect – would make a great comfy fire to heat a room.
Create a simple magical compass, using a Rego effect combined with an Intellego effect so that the target always points north. Rego Terram 10 as the total effect. Base effect 2 for slightly unnatural movement, with +1 Intellego requisite, and +1 for Touch, +2 for Ring. Makes for a great gift.
A Magus could have many of the lights crafted by a mundane craftsman, and spont cast the rings on the surface of each. This could give an almost endless amount of light for a lab with no outlay of Vis, and very little maintenance.
Simple wards to protect against vermin, humans, or beasts. Rego with an appropriate form of a level around 10. There are special rules for wards which mean they are far easier to cast too, so don’t ignore them.