A quick post to shake out my thoughts of effect level and effect power for spells which modify or cancel other spells. Specifically noting the difference between Perdo and Rego in terms of spells cast by the Magus or anyone’s particular spell in Ars Magica.
For Perdo Vim effects – I’m looking at the The Heathen Witch Reborn (from HoH:S) and Unraveling the Fabric of (form) (from ArM p161) as source examples, and I think they are both using the General spell guideline below. Both effects are at the Level+2 mags + dice, at Range: Voice. That Guideline is at R:Per, D: Mom, T:Ind as normal, then each has altered from the +4 mags to +2 mags to account for the Voice range.
Perdo Vim Guideline (p 160) – Dispel effects of a specific type with a level less than or equal to the level + 4 magnitudes of the Vim spell + a stress die (no botch).
A specific type could be Hermetic Terram magic, or Shamanic spirit control magic. A magus must have some knowledge of a type of magic (although not necessarily ability to use it) to invent a spell to affect it.
All Hermetic magi have some knowledge of all Hermetic magic.
What is interesting is that Rego Vim has several key differentiating points when compared to Perdo Vim:
- Rego defines the difference between spells cast by the Magus and spells cast by somebody else; where spells cast by others are harder to control. i.e. Suppress or control a spell of up to the (spells level +2 magnitudes) vs half of the (spells level +5 magnitudes) for each form.
- Rego does not require a separate spell for each (form) to be affected, where Perdo has a separate spell for each Art or each type of magical effect.
This is a fairly striking difference if a wizard starts customising spells; but in those spell definitions the designer must allow for the ability to affect the other spell. Elsewhere in the magic rules is a guideline which dictates that to affect your own spells at cast time is Range:Touch, but to affect another Magi’s spell is Range: Voice. This rule is why some spells are Range Touch (such as Wizard’s Boost) and some are Range Voice (such as Wizard’s Reach).
- A Perdo Vim effect at level 5 will cancel another spell of level 15 at Voice, for a specific form.
- A Perdo Vim spell can be cast at Range: Touch, but no guideline is shown; but a sample spell is as Masking the Odor of Magic. Why is it different to control or suppress a spell vs cancelling it? Does a Magus know where they cast their magic?
There it is: Perdo is the choice for lower level effects, but it can do less with the spells it affects, and has to be more form specific.
Pick your pathway to either disrupt or suspend an effect folks.