New SG, new player, need some help with the basics of spell design. This is not a question about house rules or your preferred/hated technique/form combos. Just the RAW, please...
The general guidelines for Perdo Vim include "reduce a target's Might by the level of the spell +10, as long as the spell beats the creature's magic resistance." I could find no mention, in the box on p160, that the spell created must be realm affiliated. The example that seems to be based on this guideline is "demon's eternal oblivion," which is realm specific. Is the realm affiliation need stated elsewhere, and if so, where?
I noticed that Rego Vim guidelines for wards state "a spell of this nature will only ward against creatures from one realm" (p161) but I see nothing similar for Perdo.
I cannot find a specific reference. Note that Intellego Vim also affects specific Realms.
In general, the example spells should be used to intrepret the guidelines. So I think the example spell alone suffices to require a Realm affiliation for this guideline.
The spells Dreadful Bane of the Fae and Sap the Griffin's Strength, known by Philippus Niger from Guardians of the Forest, also seem to confirm this... They affect creatures with Faerie and Magic Might, respectively.
I don't know if there is a specific guideline mention to this anywhere in canon, though...
There isn't anything stated explicitly in the Perdo Vim guidelines, however every form for Rego make mention of the guideline for the spell to be attuned to the realms. Additionally, the Intellego Vim guidelines state this as well.
So, along with the text of Demons Eternal Oblivion and the guidelines for ReVi and InVi, I think that's enough justification to make PeVi spells realm specific. Until this question came up, I had always presumed that PeVi spells were realm specific.
Note that spells are often more specific than the guideline. That's like a "metaguideline". If you interpret the Realm-affiliation as an example of this, you can argue that one can perhaps develop Might-stripping spells that are not Realm-affiliated but are limited by other means. For example, "The Ghost's Eternal Oblivion" may reduce the might of any ghost, whether from the Magical, Infernal, or Faerie realms (presumably, there aren't any Divine ones... or are there?).
Serf's parma, but (although I totally agree about the spells usually clarifying the guidelines about what's possible and not), wasn't there a bit in Legends of Hermes stating that such spells could only affect 1 realm?