If a character who was a part of the Aegis ritual is later possessed and their spirit destroyed, does the possessing spirit suffer the effects of the Aegis when crossing it's border while within the original character's body? What if the possessing spirit does not have any might (i.e. is another magus?).
Aegis affects MAGIC. It haves no effect on the body but for sure any magical creature is keept at bay unless it's strong enough to broke the barrier and tricky enough to do without destroying it (that can really ring a bell in all covenant magi).
That said there's no big gain for the possessing spirit ao contraire it can react badly at entering the Aegis raising suspicius "Are you felling well sodales? Are you sick? Can we call the herbalist? or the Physician?"
The question is tricky - but really constructing a situation where it is not resolved trivially is even more so.
So magus A participates in an Aegis ritual, and then outside the Aegis is kidnapped by maga B, who destroys his mind and replaces it with her own. She does so instantly and forever with a successful ReMe ritual far more powerful than even ArM5 p.152 Exchange of the Two Minds. So there is no magic to be registered in the combined creature. Is the Aegis, in whose ritual magus A participated, now fooled by that combined creature?
You can easily avoid this problem by not allowing that ultra-powerful ReMe ritual. Exchange of the Two Minds proper has D: Year, hence the combined creature registers as magical for its duration, and that magic was clearly not cast by a magus who participated in the Aegis ritual, hence must overcome the Aegis.
But what if you allowed that ritual powerful beyond Exchange of the Two Minds? Just what tampering with magus A involved in its ritual does the Aegis detect, if there is no magic to block? What, if after the ritual magus A lost an eye or a leg, or got a pegleg or a glass eye, and now reenters the covenant and the Aegis? There is no rule for this, so the troupe has to decide - and reading Lear (Act III Scene 4) will just help them to better define the question.
By and large I like what the previous posters have said, here's my take;
First, my summation of the spell description on page 101; The aegis dos three things:
It stops outside magic from entering, (having token doesn't matter here).
It reduces the casting score (and thereby penetration) of magic from those who weren't either casters of the aegis, have been both invited in and posses a token, or are enchanted devices that were present when the ritual was cast.
It prevents entrance by magical creatures who haven't been both invited and received a token.
So, in your case we ask if the possessing spirit took part in the casting or received a token and an invitation. The answer is, I presume, no.
I don't think that it's the most reasonable choice to assign enough intellectual processing power to the spell where it could get confused or tricked. The spell doesn't really know what a body is, it's a vim spell, not a corpus spell.
But setting the initial question aside and going deeper into how it might work, I see the token and the invitation as a way of marking an entity so that the aegis doesn't target it. Because the spell is vim I rather want to make this marking to be on the magic somehow rather than on the body or mind. That's difficult of course because the "magic" isn't really a thing that exists continually to be marked. The tokens can make this more intellectually palatable, unlike magic, a token continually exists. My model is that a token + invitation tells is a marking that he aegis uses to exclude the token wielder from its effect. I could imagine it might be possible (perhaps requiring a breakthrough) for a magus to fake this marking and allow foreign entities into someone else's aegis. It would be significantly harder than just taking the aegis down but there might still be a cool story to be told using the thought.
How does the aegis mark the original casters and those who took part in the ritual how does it mark enchanted devices inside its borders a the time of casting if their magic isn't a thing that the spell can "paint" a marking on? What about these entities is recognized by the aegis to qualify them for the "exception list"? I can see where Thespian has a real question.
I believe the only way for an outside spirit to penetrate the aegeis unaffected is for a demon to possess (without destroying the mind/soul) a person who either has a token or participated in the ritual.
I'm not sure (that's a "not sure", rather than a "no way") that this is justifiable by the rules as written. The aegis says that it keeps spirits out.
Your idea is that the infernal is in effect lying to the aegis, saying that it is the other, permitted, entity and because it is a lie it falls under the limit of the infernal and the aegis "believes" it. (Is that a fair summary?)
Yet it may well be that this is like telling a convincing lie to a wall, where you try and convince it that there is really a opening where you want to pass through it. No amount of believability can actually allow you to walk through solid rock, and even if it could the wall isn't listening and doesn't understand your language.
I would agree.
If a demon tricks a magus to handing it a token, however, this allows the demon into the Aegis.
If no being with might, no power and no effect is currently operating on a magus with compromised identity, then AFAICS we have no rule to tell us what happens when he enters the Aegis.
On a real fringe issue. Within the Aegis the possessing demon can use no Powers, not even give Magic Resistance, and have the energumen magus do nothing against his nature without the Aegis interfering.
I don't care if its written with fire in the CORE rulers book. I'm don't agree with that. Unless the host INVITES the demon there's no deal.
Also a might of ZERO is still a might (not null might) and if it's magical in nature Aegis will keep it at bay. A might of zero just tells us it will fail miserably agains the weakiest Aegis.
Btw how a might zero spirit will penetrate the magus parma?
I believe the reason for this rule is to actually make demonic possession a reasonable threat against covenfolk - it also keeps an Aegis from being an anti-disease ward.
If you don't care for it, but still want to keep within the rules, install a PeVi "everyone inside the structure gets hit with a low magnitude, high penetration, Demon's Eternal Oblivion every round" enchantment. Or put it on a gate that everyone passes through - not as good, but easier to implement. (You'll have to have some sort of Animal ward to keep possessed birds from flying over the wall, though.)
I understand your point but politely disagree.
IMHO that can be a poor and lazy excuse for a rule and has the effect of lower by a grand extent the usefullness of the second most important
ritual in the covenant arsenal.
From my understand a covenant can be (and dmust be) seen as a magical fortrees build to keep magical intrusions at bay.
The only way I can think of anything like that is by the use of the proverbial Djinn in the Bottle where a magicla being can be brough TRAPPED in a device. The important part in this example being it cannot interact with anyone or anything while trapped.
No less important:
A demon cannot silver tongue it's way inside a covenant is not worth for the hostes anyway. Those are beings made of deception and cunning if
I intrude such opposite force in the saga be sure I will role play it way in, puppeting unsuspicius magus in a long and twisted plot.
Also if I want to lazy put a magical being inside the covenant I already have some roleplay options:
The thing was already here long before the covenant
The trickster fooled one covenfolk to sabotage the ritual (maybe it caused a fire) delaying it for at least the next day and building a window of opportunit.
(for maximum drama and plot usefullness make the revelation of the intruder years after the event).
The metamorph impersonates someone wellcome.
Bait the magus outside the Aegis.
In fact if I was plotting something I tend to plot big.
Attack the most vulnerable (no parma, outside tge covenant) itinerant Mercere, bring them to your side and build a network.
Infiltrate covenants, corrupt the Quaesitoris and politicians.
Bring new powerfull knowledge to Bonisagus and let them spread it.
Enjoy a new schimist war or the domination of the whole Order.
Yes, this was the original rule that convinced me possession can fool the aegis in the first place. As I continued to ponder demonic possession it occurred to me that the original soul is also still a part of the body, and perhaps this is what is fooling the aegis. The original soul is not a part of the body in my story, so would the aegis be fooled? Yes, if the aegis targets the body, no if it targets the spirit.
I like this view. Meaning the aegis does not prevent mortal bodies and human souls from crossing it's border, i.e. the possessing magus can enter the covenant. However, their spellcasting will be effected. Could this mean that the aegis recognizes the sigils of spell casters? It makes sense. Those involved with the ritual, mystically come into contact with the aegis and it learns their sigils. Any foreign sigil will be resisted unless given a token which temporarily adds the foreign sigil to those the aegis recognizes. So my foreign wizard just needs to wait until the next casting of the aegis. Until then, hold up in your (stolen) sanctum and read for a year.
The inspiration for this whole idea comes from the antagonist of the adventure, The Traitor's Game, in TTT.
An issue which then pops up is that via Muto Vim a magus can add another sigil to his or her magic. I'd prefer that the aegis neither penalize a casting magus who uses muto vim to add a different sigil to his spells nor allow a hostile magus to cast normally when they add the sigil of a protected magus to their spells.
I don't agree with this ruling either. Aegis is a ReVi ward that prevents creatures of Might from crossing the boundary. The demon is crossing the boundary, whether it's riding within someone or something else or whether it's moving under its own volition.
The justification for the RoP:I rule is probably that demons are undetectable to Intellego but I find that unconvincing in this case. There's no suggestion anywhere else that Aegis has any more intelligence than a wall does.
There's a case to be made that undetected demons should be immune to any Hermetic Magic, in order to be truly undetectable. I've made that case before. But that's not RAW. If spells like area effect PeVi work on sneaky spirits, Aegis certainly should.
Generally, the rules for undetectable demons are not well thought out, IMO. One more item that needs revision if a 6e is ever developed.
It does not mean, that MuVi can fake a sigil for an Aegis, though.
These traces may indeed also be detected and compared by an Aegis, but they should contain more information than the form, by which a magus detects and identifies their caster.
How can a magus, who has never in utter detail analyzed one of caster A's spells before, but just knows that A's sigil is "the scent of orange blossoms" or perhaps has seen A cast a spell some time, fool an Aegis which has incorporated the exact traces A left by helping to cast it?