Okay, I am sitting at home looking at the core rules.
Jebrick is right. The description of WoMS does indeed say that it “does not affect spells of a Momentary Durationâ€.
This is a point against me. Sloppy. This is why I put up a Perpetual Smurf Parma a while ago. The word “forbid†is never used, only that this particular spell cannot be used the way that I stated for countering Momentary spells (which is what you want 90% of the time when trying to fast cast a counter spell).
For reference, as someone stated earlier, 4th edition WoMS states that it “does not affect spells of an Instant Durationâ€. This says to me that the intent of the wording was to say you couldn’t undo ritual momentary effects. However, we are not trying to second guess the writers here, just what the text actually says. I think it was an oversight, but I cannot prove it for absolute.
UtFo(F) has no such stipulation. This spell is new to fifth edition, not revised or rewritten from an earlier edition. It carries no legacy baggage. This is a point for both sides. It does not contradict that Perdo Vim can be used as a counter, and it does not contradict Jebrick’s assertion that you must rely on specific Form spells. It affirms neither, but it is a place of refuge for both sides.
Now, let us look at Base Formulae. The Guidelines give us three different formulas, two based on Canceling an opposing spell and one based on Reducing an opposing Casting Total. WoMS, that old legacy, uses a different formula not listed in the guidelines, and the guidelines mention nothing about Momentary effects. So that makes 4 formulae altogether.
(Paraphrased) They are…
[color=white]……..1. “Dispel effects of a “specific type*â€; if you can roll higher than its level on a stress die + (Base Level + 4 magnitudes)
[color=white]……..2. “Dispel any magical effectâ€, if you can roll higher than twice the casting total on a stress die + (Base Level + 4 magnitudes)
[color=white]……..3. Reduce an opposing casting total by half (Base level + 2 magnitudes) of this spell, provided it Penetrates the magic resistance of the opposing caster. This one is strange, there are no precedent or example spells based on this formula.
[color=white]……..4. The Legacy Formula in WoMS, Cancel the effects of a spell; as long as you can roll twice its level on a stress die + (Base Level + 4 magnitudes). In the case of WoMS, the 4 magnitudes are used on Range (+2 Voice) and Target (+2 Room). Let’s not touch the Room part, that is a subject of debate in another current thread. Now, notice that this combines Guidelines # 1 & 2. It bases the ease factor on the level of the opposing spell like #1, yet like #2 it requires it to be twice as high. Compared to #2, it has an advantage against low level spells where the casting total is usually higher than the level, but it is disadvantaged versus higher level spells where the casting total is usually lower than the level.
Something in the Guidelines is wrong, or something in WoMS is wrong (besides Room).
But I hope I can get everyone to agree on two points..
First, that Perdo Vim of the “appropriate level and design†is good against any magic,
And second
If it is of the appropriate level and design, a Formulaic Perdo Vim spell can indeed be fast cast to counter an incoming spell.