We have a magus who has a Guardian Angel virtue. It finally occurs to us (me, anyway) to wonder about angels and interaction with an Aegis. Obviously, it would have no difficulty with the mage's home Aegis, but what about foreign ones? It seems unlikely the angel (or any angel) would be halted from its divine mission of attending to its charge, and does not need a token to enter an Aegis.
My conclusion is that a guardian angel ignores the Aegis while in spiritual form and attending its mortal charge, and can advise as normal. If this is so, it probably is inhibited from other functions.
However, I might be wrong, and I could be persuaded. Opinions?
RoP: The Divine doesn't directly speak on it but page 22 provides answers.
"An angel in purely spiritual form can only be affected by Mentem or Vim"
Furthermore, it states that magic cannot summon or compel Angels (at least not Magic magic).
In my opinion, warding and compelling are two different things. Angels should be affected by the Aegis of Hearth.
Though we shouldn't forget the cardinal rule of the Divine realm: God can just say "nope" and let the angel through. Obviously that shouldn't be norm, but if the storyguide thinks the advice the guardian angel has is high profile enough, it could definitely happen.
In general the aegis should keep angels out, the only caveat I would have is the question of whether inviting the mage could be seen as inviting the guardian angel by extension. Even pre-Christian warding magic would likely have allowed someone's family Genius to accompany them. On the other hand the description of the spell indicates only specific known entities may be invited so if such an allowance is made it would most likely be in the archaic wording of a formal invitation "Mage Honorius, I invite you and those who watch over and protect you unseen, enter freely into our covenant and..."
Perhaps with more caveats about bearing no ill will to the covenant and its occupants or something.
Ultimately, the Aegis as written does keep "generic" angels out, but we probably do not want it to block the guardian angel of anyone who's invited. At the same time, if we allow the guardian angel in, we should probably allow in a Ghostly Warder, a Supernatural Nuisance etc. too.
If so, I probably find Silveroak's proposal the best direction to look to. I am not quite sure what the wording would be. I'd like something that sounds like it's inviting just the magus, with extra words that are easily dismissed as just quaint tradition and ceremony, but upon careful inspection effectively do invite guardian angels, ghostly warders, supernatural nuisances, familiars, and any other beings similarly "tied" to people who've been invited in.
While Dominion is a very active agent in Mythic Europe (and beyond), God is not, and I think a DM should be careful about using direct miracles from God just illy willy when the easiest solution is for the Magus with Guardian Angel to just invite him in.
It's just like Star Trek. You can beam through shields whenever it is dramatically appropriate. To show how difficult it is, you add the concept "interference". That means something like the angel appears to be half-faded or appears more slowly (it modulates its phase variation to the Aegis frequency or whatever).
Roleplaying games are about telling stories, not about nitpicking.
Not sure I agree with that. While I don't run a super crunchy Ars Magica game, I think Ars is a system that rewards a strong understanding of the magic system. Players like to be able to design spells knowing exactly what those spells can and can't do - as a storyguide, ignoring that for the sake of "my story" doesn't sit right, without at least some in-game justification
"Within a Host" is the important distinction here, much the same way Goetic Binding makes a spirit to be, essentially, a part of the target. There is another precedent for a Magic-bassed possession that functions in a similar way. But again, this is only applicable for the Possession power.
If any of these were to be general capabilities of Demons, Angels or Magical Spirits than you would have 3 realms that can simply ignore the Aegis, at which point...well, what would be the point?
But that they're all circumstantial abilities makes them more managable. Divine creatures are warded, unless God himself intervenes. Magical and Infernal Spirits are warded, but could circumvent wards via their Possession power.
To clarify, for guardian angels I am making the proposition that GAs specifically have been assigned to that person by God Himself and that they cannot be compelled to depart or otherwise cleaved from their charge, and certainly not by flimsy magical barriers. I'm not asserting that angels can generally walk through an Aegis at will, although others might be. Furthermore, I suspect that the angel may be suppressed or at least constrained while in another Aegis.
I kind of like the idea that various ghosts, faeries, and other supernatural pests haunt the boundaries of covenant Aegises (Aegi?). It makes them creepier places and gives reasons why some companions find magi and dwell in their settlements. Most supernatural entities should be excluded by an Aegis, barring overwhelming might.
They are about telling stories, but I like to think I follow the setting's world rules better than most TV series. I like RPGs to have allowances and constraints. Constraints are good in fiction and in role games; they promote creativity and can lead to explorations of or toward solutions.
I would expect Guardian Angels to have a similar ability and be one with their charge. That does not mean that they are not impaired. Demons, Angels, Spirits, while in possession "State" probably have very limited abilites other than being carried by the host/charge. As such, the Aegis still plays a significant role.
A pocessed person might say that the voices have stopped. Some demons might prefer leaving the host rather than risking being "trapped" in an Aegis...
I think it is possible to ward against angels if God isn't willing them through. At least, that was my plot point for why the angel dropped the Dannebrog over the battle of Lyndanisse in 1219 (the angel couldn't fly on to the battlefield, but could drop a holy relic through the barrier from a great height). I also used soldiers from Rugen having Second Sight so they could see through the illusions woven by Muspelli to conceal soldiers.
I feel like this should vary depending on the saga taking into consideration whether magic and the divine are in tension with each other. Quite a ballsy thing for a mage to fully forego the Parma in exchange for his guardian angel.
... and think about the Guardian Angel during the apprenticeship of the magus. That becomes a five volume novel neither a magus nor a monk wishes to read.
OTOH: How would a full magus with his Parma usually up acquire one? Wouldn't that rather boceme a Major Story Flaw: Plagued by an Angel?
Gust of wind to make you look, turn the pages to the right one
Knock on the door
Note on the desk
Possess a devout for a minute to give a message
Slip under your skin at Sunrise/Sunset