I've always played that when a spell is cast upon a magus inside of an aegis from a caster outside of that aegis the magic resistances from the Parma Magica and the Aegis of the Hearth stack.
In other words if a character with Corpus 5, and parma magica 2 is siting inside a covenant protected by a level 20 Aegis of the hearth and some opponent casts a range arcane connection version of clenching grasp of the crushed heart on her. The attacking spell would need a penetration of >35 to work. Not just >20.
I don't think that this is supported by the text of the spell in any edition. Has anyone else played it in this fashion?
That would be my take. but then, one might argue that a penetration high enough to breach the Aegis would then carry on to test the target itself alone. So if the Aegis is 20 and spell penetration is 21, it gets through the Aegis and the Target would have to have greater MR than 21 to avoid suffering the effects. I see justifications for both views really.
I suppose it can also be argued that those protected by the Aegis are usually those party to casting it in the first place. So, in some mystical arcane fashion their own MRs are tied to the protection of the Aegis itself and thus stackable.
I would say you should add them together.
This way casting from outside gets a penalty equal to the level of the aegis.
But if you are within its borders your penalty is only the half of it to the casting total.
Going by just the words on the page, I don't think they stack.
Remember, there are two different effects at work.
Any spell cast from outside the aegis must penetrate the level of that Aegis. If it is strong enough to penetrate, the spell carries on unabated and the target magus must rely on his aura-adjusted magic resistance.
But a magus within the aegis takes a penalty to his casting total (my understanding is that the aegis must first penetrate that guy's resistance...), which of course then effectively "stacks" with the target's resistance as he has less to resist.
But I think it kind of balances out. If you want to cast a spell at a protected target from outside the aegis, you'll probably need an arcane connection in order to affect him, which pushes the level of the spell up a magnitude or two.
Of course, if you have an arcane connection, your penetration gets boosted too.
I don't think they are meant to stack. In my saga we play that they don't stack.
I would play that it only needed a penetration of > 20.
You would bother because the Aegis protects everyone in the Aegis; not just the magi. Without the Aegis a hostile covenant or hedge wizards or faeries or whatever could just kill all your grogs (or companions or apprentices), or Mentem magic them into attacking the magi or something equally annoying. Or use Terram effects to attack the covenant itself.
The Aegis is also obviously useful if the magus has a lower magic resistance from Parma Magica + Form than the Aegis. In Erik's example the magus alone has a Magic Resistance of 15, and the Aegis has a value of 20. This would not be untypical for newly apprenticed magi, who were casting the Aegis themselves. The Aegis could be considerably bigger if an older magus was casting it.
Of course, as the magi age the balance of which is best might change, depending on how much effort each magus puts into learning Parma Magic and how many resources the Covenant puts into the Aegis.
As I said in my original post I don't think that the rules ever state that they stack so it's pretty clear that they don't.
On the other hand I've been playing this way since I first used Aegis of the hearth in a game (that was a very long time ago).
I've got some responses saying that they would play things my way.
Has anyone played one way or another and had it make a difference. Have their been antagonists that pegged PC's through their Aegis where my rule would have prevented it? Have the PC's blasted their opponents in their opponent's covenant where my rule would have stopped them?
In my saga we've had a bit of scrying both by PCs and NPCs through Aegii that wouldn't have worked if Aegis and Parma stacked.
On the other hand, some of the scrying would have worked in either case, as the targets were either apprentices or grogs. And when magi were scryed on, much the same effect could probably have been achieved by scrying on a grog instead.
We've also had a covenant beseiged and overrun by some Tremere and Flambeau. I wasn't the storyguide for this, but I suspect that it wouldn't have made much difference as the initial attack was mostly directed against the covenant infrastructure rather than the magi.
So, I guess in my experience, I don't think it has made much difference.
I think that the main effect would be that if Aegis and Parma stack, then sitting tight in your laboratory for the season is a good option if someone declares Wizard War on you. If they don't stack, then you are still likely well protected in your Aegis, but not so nearly invulnerable. Not stacking makes Wizard War scarier.
I have it not stack, as that's what the rules seem to indicate to me. I also like the way it plays that way. It means that simply casting the Aegis alone is not sufficient for real protection unless you cast a really powerful one. It means that covenants are defended, but not invulnerable.
Fighting inside someone else's covenant is still going to be bad news. I guess I've always seen the main function of the Aegis being in repelling supernatural intruders and in protecting the covenfolk and buildings, not specifically in protecting the magi themselves directly.
We seem to assume that it does not stack because it does not state that it does stack and I am inclined to treat the magic effect guidelines as exception based given other interpretations (one of my bug-bears is the circular ward effects).
That being said: the Aegis of the Hearth is already fairly exception-laden - so maybe it is acceptable to take it a little bit further and allow it to stack?
Especially concerning whether to hole up in the Sanctum during Wizard's War.
I see two situations. One where the enemy tries to cast by Arcane Connection. The second, where the enemy sneaks inside the Covenant, and casts by normal means.
While inside, the casting total is penalized by half Aegis level, right? Should things be easier or harder, for someone casting by ways on AC through the Aegis, than inside?
I believe we do both situations the same way. Any spell cast at a target in an Aegis, by any way, has casting total penalized by half Aegis level.
So you don't have it count as magic resistance ever? My understanding is that its MR when cast from outside and a penalty to Casting Total when cast from inside. Which is more effective depends entirely on the situation.
If the target is the buildings or the mundanes, giving them all MR is definitely better than just penalizing the caster. Its really only when the target already has MR that you want the casting penalty.
Yes, well - I have to confess, we have a house rule. It's actually about the use of devices inside a foreign Aegis, but the philosophy of it can be stransferred to spells cast at Aegis as well.
So we rule, that a device in a foreign Aegis has its Penetration penalised by half Aegis level. Any device thus having negative Penetration, will not work.
I'd say the same for spells. They get a penalty to casting total equal to half Aegis level. Effectively, this lowers Penetration as well. So a spell with negative Penetration, will not affect even mundane peorple and objects with no MR of their own, inside the Aegis.
