AotH does seem to share characteristics with the intended effect. Guess it could fall into the magic resistance limit ... hence the required breakthrough.
I liked the MuVi idea of making the effect resistant to dispelling also...
The discussion about perceiving the target is also interesting. PeVi is best to get that effect but the individual can still target himself (individual target) and rid himself of effects that he does not per say perceive.
Thanks all for the discussion.
W
Yep. Doesn't seem (to me) to be problem?
Yep, and magi don't spend all their time indoors.
Of course, but that is Magi Solving Problems With Magic. That's what magi are meant to do. And although that should be effective, it's not totally infallible, and nominally the Aegis keeps nasties out of the covenant anyway. The demon detecting item still doesn't help if the demon's aren't in the covenant/castle, or if the item has been sabotaged by infernalists, etc. There is still plenty of scope for Magi Hubris To Be Defeated By Clever Demons.
But you can just detect demons by carrying relics. If the demon's approach within the scourging aura of the relic they get enveloped in a shower of sparks (or other special effect).
It does make sense to me to compare the OPs suggested effect with Aegis of the Hearth. After all, it is a protective effect, cast in advance, in case it is needed.
Requiring this effect to be a Breaktrhough seems plausible. Also, Aegis is a Ritual - so should this effect also be? Perhaps. But does Aegis need to be a Ritual, or is it simply because of the D: Year? Given the recent discussion of risks of Botching Aegis, it is perhaps best to not have to cast it every month. Also, magi are busy people, and they may not want to leave theor labs and libraries, or they may be away. And it's nice to have protection in place for a long time. It may also be a balnce issue from a game-mechanics point of view. Something as useful and powerful as Aegis requires a sacrifice, in the form of vis. Because it seems that a lot of people don't like the Botch risk and feel even a single level of Mastery mitigates it all.
This thread appears to be bifurcating, and since I find both parts of the thread interesting, I thought I'd reply to both.
Thinking about it however, I find that my answer in both cases are "I agree with Richard Love".
Protecting a spell (against another spell) does seem somewhat like more like ReVi, if it is to protect against T: Room/Structure style effects.
Some for of CrVi shell like effect would probably help against T: Individual Unravellings, but not the more general case I think.
If you're going to cast an Aegis with different parameters, you certainly will, according to the last paragrafs of ArM5, p. 161
Alternatively, T: Boundary will do as well. And inventing an Aegis with any other parameters will require a serious breakthrough, as already referenced.
With all due respect, now you're being silly.
The reason we worry about botching the Aegis is/was the use of Vis. If you've somehow made a non-ritual version of the Aegis, it obviously falls under "Formulaic Magic", and as such can obviously be reduced to 0 botch dice through all of the usual means.
And ofcourse, you'd be able to generate enchanted devices creating either Aegis-like bubble, or simply granting MR, neither of which I am a fan of.
Now, returning to the T: Room DEO version, I again agree with mr Love.
I don't see the problem? It's not like you can (trivially) know when they are inside and when they are outside, no?
And in their spiritual form, they can ignore more of the reasons that would cause humans to go inside, so there's only really temptation left, neh?
The only thing that worries me here is that you appear to assume that MuVi can change a demon into something detectable somehow?
The only MuVi guideline I can think of which deals with demons, binds them to a person or object (RoP: Infernal, p. 121, box). Which seems to open the caster to all sorts of accusations of diabolism.
Agreed, at least to some (large) degree.
Agreed, so make the most of it.
Eugh. Then you might as well play That Other Game, really. 
Yes, yes I am. I thought that was a given fact. 
Apparently I completely disregarded the fact that Aegis in RAW qualifies for a Ritual due to both D:Year as well as T:Boundary. Reducing only one does nothing significant. And that the Botch risk is for vis use, so a non-Ritual Formulaic spell is not that risky at all.
I stand corrected. All respect duely noted.
Nope. Me neither. let's not do that.
I didn't intend to turn this thread into a rehash of the old discussion about demons, so I'll just make a couple final comments.
I believe it was an extrapolation on the MuMe guideline from TMRE that makes spirits corporeal, to do the same with demons and (I believe) to turn them bright blue or something else similarly silly and obvious. Since Vi spells only work on a specified realm it wouldn't trip up other spirits.
The alternative was ReVi to force all infernal beings to loudly announce "I serve Be-el-ze-bub", preferably in a high-pitched voice.
Either way you get a seemingly effective demon detector. I sure there are other options too.
Take a look at "The Limit of the Infernal" on AM5 pg. 80. You seem to agree with the optimists who think the limit derives from a flaw in Hermetic theory, since in your interpretation it only applies to Intellego and can be circumvented with magic based on other Forms. To me the moderate or pessimistic views make demons more interesting.
My interpretations, almost assuredly non-canon.
I guessed that was probably it. I don't think it applies, but accept that the extrapolation isn't insane or stupid.
Firstly, as mentioned, I don't think it would work.
Secondly, I think that the physical manifestation would be something that the demons would be able to lie about. And demons lie Perfectly.
I've played in sagas where this would get you Marched as a diabolist, even if you then proceeded to DEO every one of them.
After all, you've just proven that you've capable of commanding demons.
Sadly, yes. Ish.
Take a look at "The Limit of the Infernal" on AM5 pg. 80. You seem to agree with the optimists who think the limit derives from a flaw in Hermetic theory, since in your interpretation it only applies to Intellego and can be circumvented with magic based on other Forms. To me the moderate or pessimistic views make demons more interesting.
[/quote]
I'm fascinated with being called an optimist here! 
It is a new and thrilling experience.
Firstly, the Limit of the Infernal as you quote, explicitly refer to Intelligo Magics. From this, there's no reason to assume it pertains to any other Technique. Though you might want to see above with regards to 'lies' and 'lying perfetly'-
Secondly, the very existance of DEO indicates that magic can separate demons from other things. Arguably, it is all the demon detector you will ever need. Clever demons put you in the situation of never casting DEO on them in the first place. Or any other "demon detector" for that matter.
Ok, one more very quick comment on demons, with apologies to the OP and his thread on magic resistance.
We all have to be optimistic about something! If the limitation really does apply just to Intellego, then there's clearly a flaw in that technique, as you optimists argue and those magi who believe the problem is either due to the Will of God or the Essential Nature of Demons are being a little silly.
I like your ideas about lies with regard to Mu and Re, but as you say DEO itself is all that is really required to separate demons from other things, with the minor exception that it also targets non-demonic Infernal entities such as ghosts or djinn. Unless those count as demons anyway; I'm not sure of this. Hence my suggestion that magic not target undetected demons unless boosted by arcane connections, regardless of theoretical area effects.
But how will anyone know if an unsensed demon has been affected?
I think a tree just fell in the woods somewhere.
I assume we're talking about DEO here. Cosmetic effect, maybe? It's actually one of the reasons I steered away from DEO in my own examples, since the effects are less clear than we might wish.
I suppose one could introduce a subjective rule of "If it looks like an Intellego and quacks like an Intellego than it IS an Intellego" and leave other area effects unchanged, but this opens other cans of worms.
It rather does I think.
I tend to assume there isn't one. Combined with not accepting the extrapolation of using MuVi to force a demon into manifesting...
You can have demons in siritual form be affected, but no-one will know (unless they can sense spiritual forms).
RAW is kept (T: Room/Structure), and no-one the wiser.
Really, I think the problem comes from the MuVi extrapolation.
I'm back on the forum! Glad to see you all again Maybe I'll even get back to that 400-year-old Sahir project!
Not without a Breakthrough, in my opinion. In my view, a "protective shell that blocks all incoming magic below level X" is effectively granting Magic Resistance. We know that that even Hermetic Magic can do so only with difficulty: Parma can do it, an Aegis can do it, but any other solution canonically requires a Breakthrough.
I don't think one can draw the inference. There's a difference between obscuring, and protecting. The CrVi guideline in the book is like a wall of fog: it blocks sight, but not arrows (although it does make targeting harder!). The CrVi guideline you propose is like a wall of clear crystal: it blocks arrows, but not sight. The fact that you can conjure either one does not imply that you can conjure the other - think of what you can do with Auram.
Demon Detection: since the thread has already been derailed by someone else, I may as well spend two words on it 
I think the problem can be summarized as follows. We are told that Hermetic Magic can't force truth out of a demon, so using Intellego to detect, mind read etc. a demon is in general ineffective. Similarly, one could argue that using Rego to force a demon to tell you the truth is ineffective. However, since many Vim effects are Realm-specific, you could simply use on that innocent-looking lass a Perdo (Infernal) Vim "eternal oblivion" effect, or a Rego (Infernal) Vim "jump up and down" effect. If she disappears in a puff of smoke or starts jumping up and down, she's Infernal. Arguably, this does not necessarily mean she's a demon - she could be a demon child, or an infernal ghost - but it seems that revealing Infernal association is already more than Hermetic magic should be capable of with a demon.
Well. My troupe and I have spent a lot of thought on it. This is how we deal with the issue after reading RoP:I.
First of all, note that every demon has a strong temptation to deceive, but that doesn't mean it will always deceive. Sometimes that's because it manages to keep its temptation in check in order to create more mischief. Sometimes a demon's other vices just have the better of it: maybe a demon is just too proud to try to pass for a mundane human, and another resists Intellego interrogation but it's too slothful to oppose a Rego Vim "I command thee" effect. This explains why you do run across demons on which "indirect detection" will work - and even some on which "direct" detection, or a plain question, will yield the truth!
But what about those for which deception is the top priority? Well, they deceive you. They are, in fact, so good at deceiving, that they've tricked not just characters, but players and Ars Magica authors too. Remember all those "dark" faeries, those "chthonic" magical spirits that appear unholy to Sense Holiness/Unholiness, but really are just creatures of Faerie and Magic? 
Dismissing the demons for a second, would the below work?
MuVi 20 - Touch | Mom | Ind | - Strengthening the Intangible Bonds (Vi)
Targets a Vim effect of up to level 40 and with an added casting requisite, modifies its structure so that dispelling effects that are intended for a specific type (form) of magic fail to dismiss it. Dispelling effects that affect all magics such as wind of mundane silence work as normal. Laszlo of Ex Miscellanea used this effect to prevent the dispelling of some of his low level effects after a plan his his failed thanks to a well placed spontaneous dispelling effect.
(Touch +1 | Mom +0 | Ind +0 | Base 20 (general +1))
W
Stepping away from those pesky demons myself and trying to contribute to the original thread...
How about the MuVi guideline "Significantly change a spell of less than or equal to the level +1 magnitude of the Vim spell" such that it unravels as Form X rather than its real form? The actual Form doesn't change, just the way it reacts to dispelling magic.
How many Magi are going to figure out that the eternal flame is best dispelled with Unraveling the Form of Aquam?
Kinda what I was aiming for but with the superficial guideline. Always have trouble getting the right level.
Significantly is defined as +/- one magnitude or a change in target. Guessing you are implying that changing the nature of the spell from being affected by PeVi vs fire to PeVi vs Water is similar to a change of target and as such warrants the "significant" tag? Then again, the effect does not change the power or level of effect...
Game balance wise it does not change much as there is no real question on penetration for the effect..
W