Oh, you weren't that's why I used the smiley!
I agree that this is the case in general.
Although it is also important to note that once the Aegis gets to a high level you will actually need a ritual version of Unravelling the Form. Looking at the table calculated by Jonathan.Link earlier, you can see that once the Aegis gets up over 60, that the corresponding Unravelling spell gets up over 50, which means it is a ritual.
So, a big Aegis does effectively need a ritual to dispell it.
I don't get it: why, because the SG wants to, would always the NPC succeeds?
If there are rules for casting a spell, then NPC can fail to cast it.
I read your "if SG wants that NPC do it, they do it" as a "well, rules are only for PCs". Then why even bother with dice rolls? The SG will just decide when the PCs succeed...
If not, then the NPC can fail. I can see stories in both sides:
- the NPC succeeds ; PCs will check, understand, prepare to counterattack, etc.
- the NPC fails: they will try another method. They will try to get an arcane connection.
Both of those are interesting stories, and the die roll make the story which will be told. Why decide in advance what the story seed will be?
And when my character wants to do the same trick?
Well, your character should fail, naturally. Otherwise how would he learn anything, othe than by setbacks? 
Kidding aside, it's vital IMHO to find some sort of balance between rules-driven rulings and story-driven ones. It is annoying to be following the rules as a good little player and have something contrary tot he rules happen. You feel shafted. OTOH I agree than some things whoich aren't very well defined - likes this very Aegis question - should not be trivial events but Story events.
A possibility could eb to rule that as Aegis subtracts half level form casting inside it so does it affect spells cast on it or somehow interfering with it. Like Teleporting inside it. Or Unravelling it.
Not really. Unraveling the Fabric of Vim with Mastery 3 and a choice of Unraveling (not restricted, just written in one section) can handle that pretty well. So it does get increasingly more difficult, but there isn't an effective need for a ritual.
Chris
Well, unravelling is always adding a stress die.
Going by that route, you can statistically cast unravelling level 2 (because touch should be mandatory) formulaic, and hope for a 1 1 1 1 10.
This would totally increase your penetration.
If you spend 1 hour casting that spell, it means 600 attempts. On 10 hours, 6000. One would be a 40 with chance.
Such a character capable of doing this would be highly focused and specialized. The OP suggested that it was relatively easy to take down an Aegis. I've been countering that it is not, mechancially and story wise. You present a case where it becomes easier for an individual character to do it by adding some mastery to the mix. A specialized character capable of doing this can certainly exist, but they may not be easy to find or interested in doing this particular task. The trade off you present is one of specialization versus needing to cast a ritual versionof unravelling. Neither option is inexpensive or easy.
Right. I agreed it gets progressively more difficult. I just disagreed on how such an Aegis level would push us towards rituals so quickly because there is a non-ritual alternative.
Chris
I don't think that I'm familiar with that Mastery. What does it do?
It's specifically for Perdo Vim spells. It adds 3 times the mastery score to the level of the spell when determining whether or not it can destroy the targeted spell.
OK. I see. Thanks.
Well, that doesn't really seem to make much difference. It still means that you need a ritual to destroy a "big" Aegis, it just puts that threshold of "big" off by about 10 levels.
What book is that Mastery in?
Introduced in HoH:Societas.
Heres a Mastery Index I found useful: germanitas.org/rpg/germ/down ... astery.pdf
I couldn't find it on the Atlas site, but I didn't look all that hard.
Because if the story-guide wants a character capable of doing X, he just needs to make up a character capable of doing X. So, if the story that the story-guide has in mind needs an NPC with lots of vis, say, he makes up a way for that NPC to have lots of vis.
Of course, but the story-guide can arrange for the NPC to have the resources (Art Scores, vis, friends with Wizard's Communion, whatever) available to cast it. If the story-guide wants to.
Of course, the story-guide might want to tell a story where the NPCs don't have the resources to do what they want to do. Which is fine as well. But either way, the story-guide definitely chooses what the NPCs can and cannot do.
Well, sort-of. If you're a PeVi caster, it's pretty good to have several levels of Mastery for a spell like that anyway. You'd definitely want Fast Casting and Unraveling, probably Penetration, and maybe Multiple Casting. There are a few others I would definitely consider. Knowing that you could drop a confidence point (or two) into it and that you could try a few times, especially if you also chose Multiple Casting, then you start expecting level 50 + extra 10 + Unraveling/Confidence 15(ish) + roll 10(ish) = 85(ish). That's well over Durenmar's wartime Aegis. However, the real issue isn't the ritual, it's that you also need to penetrate, so the required Casting Total plus Penetration rises at double the rate at which the Aegis rises.
Still, even without Unraveling you still don't need an Aegis. Ask any decent PeVi Verditius to do it for you (or be one yourself). Build a level 75 Unraveling the Fabric of Vim with 76 Penetration. You could have it be a charged object since you probably won't have to use it many times.
I'm not saying you couldn't do this with a ritual. I'm just saying there are a bunch of other ways - you certainly never need to use a ritual.
Chris
level 65, pen 76 - unravelling works at +2 magntudes.
Lab total required: 103 or so. Doable, but not by just anyone.
This is rapidly approaching something that needs a life-time (or at least decades) worth of dedication for a magus to achieve. So, it is hardly something easy or something that most magi will have a hope of being able to do.
I think this is proving my point. IT's easy to design a character that could Unravel an Aegis.
It is not easy (or cheap) for the character to reach that point.
For a very high level Aegis, yes, it takes more effort. But it's really not that hard for a Verditius to take down a level 50 of so Aegis. Here's what will pull it off:
You need a level 40 spell with +50 penetration, so that's a level 65 effect.
Int 3 + Aura 3 + Craft 5 + Magic Theory/Philosophiae/item 6x2 + Item-bonus-lab 9 + Perdo 12 + Vim 12 + Assistants 9 = 65
How really PeVi-dedicated is this Verditius? We could certainly boost the Assistants bonus easily. The Verditius could have a familiar, an apprentice, forge-companions, and the magus for whom he's making the thing. It's not like he's got a crazy Aura. His Craft is only 4+specialty. His Magic Theory is only 5+specialty, and his Philosophiae is lower. It only takes about 5 seasons or so to get there without a very big lab, and with assistants that could go faster. Raise a few of these a little and drop Perdo and Vim, and you could even quickly get to the point of Perdo 6 and Vim 6, which really is starting to say that nearly any Verditius who could train an apprentice ought to be able to pull this off without even focusing much on Perdo and Vim.
Chris
But would a Verditius do this?