Hi,
having more time this day, I'll answer more completely. I hope you find this answer useful.
Consider a Spring covenant. The magi there cast Aegis of the Hearth 20 with a casting total 32, so it has a Penetration of 12.
Three faeries try to enter the covenant.
First, a Brownie of Might 5. It can't enter as the Aegis penetrates the Brownie's magic resistance and the Aegis is above the Brownie's Might.
Next, Sir Excelsis, Might 25. It can enter as the Aegis is below the faerie's Might.
Finally, a Mormo, Might 15. It can enter. The Aegis is above its Might, but the Aegis has insufficient penetration to affect the mormo.
Yes. I read in Arm5DE: "No foreign supernatural creature with Might, of any type, can cross the boundary of the Aegis unless its Might exceeds the level of the Aegis. The Aegis of the Hearth must Penetrate the creature's Magic Resistance in order to have this effect."
Once inside, Sir Excelsis uses its Damaging Effect power (2 points). The penetration would normally be 25 - (2 × 5) = 15, but the Aegis subtracts half its level from this (i.e. 10), so the damaging effect has penetration 5. The weapon is likely to be stopped by any magus's Parma Magica.
The mormo uses its Possession power, spending 2 points. The penetration of the power would normally be 15 - (2 × 5) = 5, but the Aegis subtracts half its level from this (i.e. 10), so the Possession has penetration -5, or zero. Enough to affect a companion, not enough to affect a magus.
Totally agree: "Any foreign supernatural effect originating within the boundary of the Aegis must subtract half the level of the Aegis from any Casting Total or equivalent. If the effect does not have a Casting Total, as for an effect from an enchanted item, then half the level of the Aegis is subtracted from its Penetration. If the effect has neither a Casting Total nor a Penetration, like the Parma Magica, it is not hindered by the Aegis once it is inside. The supernatural powers of creatures with Might that have been able to enter the Aegis are affected in the same way as any other foreign supernatural effect; this effect does not need to Penetrate Magic Resistance, unlike the exclusion effect."
Later, some magi become interested in the covenant.
Alice the maga has Parma Magica 3, Penetration 4, Vim 12, and a bunch of spells. She wants to spy on the covenant. She casts Eye of the Eagle (InIm 25) with a casting total of 30. That gives the spell a penetration of 5 + 4 = 9. This is lower than the Aegis level, so she can't see past the boundary.
I agree for the calculation if it were for example a "summoning the distant image" spell (which is also InIm 25 but with range arcane):
"This also applies to an existing foreign supernatural effect that tries to cross the boundary. If the effect cannot Penetrate the Aegis, it is dispelled"
For the fact that In spell need or not to penetrate, I would agree with other posters that this spell is affecting you (range personal) and that it's your sense which is improved so I disagree with the fact that this particuliar spell is affected by the aegis.
Alice really doesn't like a magus in the covenant and decides to cast a variant of The Wound That Weeps with an Arcane Connection range. Her first attempt at casting it generates a Penetration total of 18 (including sympathetic connections). This doesn't penetrate the Aegis. Her second attempt is more successful and generates a Penetration total of 22. This is enough to Penetrate the Aegis.
See my first answer also.
Here I disagree: the sympathetic connection bonus to penetration only counts for penetrating magus target. It doesn't count to penetrate aegis target. I could see (see another poster's post after my first) having a aegis token as a sympathetic connection bonus to the aegis itself (of the "indefinite" version [understanding that the aegis only lasts for year]) for the purpose of penetration, but not a vial of blood of magus target or the familiar/talisman of magus target.
See my first post for what I would decide.
The target magus has a Corpus magic resistance of 15. Does the spell affect the target? If the Aegis adds to the target's magic resistance (20 + 15 = 35) the spell won't affect the target. If the two magic resistances have to be overcome separately, it will (magic resistances of 20 and 15 are both lower than the 22 penetration).
There is no addition.
First question do the spell penetrates the aegis: yes no. 21 penetration is required.
Second question, does the spell magus target: I assume that 15 is their full MR and that you didn't forget the parma magica x5 in the calculation, nor any magic resistance mastery. So 16 penetratiopn is required.
If spell has penetration >= 21, all good. If not, miss.
Alice decides to sneak into the covenant. She casts Veil of Invisibility on herself and walks in. Her Vim magic resistance is 15 (Parma) + 12 (Form) = 28. The Aegis's penetration (12) is too low to affect her. She enters, still invisible.
Yes: "This also applies to an existing foreign supernatural effect that tries to cross the boundary. If the effect cannot Penetrate the Aegis, it is dispelled. However, if the spell is cast on someone with Magic Resistance, for example an invisibility spell on a magus, then the Aegis must Penetrate the magus's Magic Resistance in order to dispel the effect. This includes such effects as the Parma Magica, Hermetic Longevity Rituals, Familiar Bonds, and Talisman Bonds. However, the last three of these are automatically sustained by the underlying magic, and so immediately come back into effect once the magus has entered the Aegis. Parma Magica must be raised again, in the normal way. "
If Alice had MR less than 11, then we should see if her invisibility spell has enough penetration (> 20) to be kept when she enters.
Now inside, she tries to cast Eye of the Eagle again. She gets the same casting total as before (30) but now subtracts half the Aegis (i.e. 10) from this. She successfully casts the formulaic spell at the cost of a fatigue level.
Yes.
Alice is followed by Bob the newly gauntleted mage, also with Veil of Invisibility cast on him. Bob's magic resistance is 5 (Parma) + 5 (Form) = 10. The Aegis will penetrate Bob's magic resistance. The Aegis may dispel the Veil of Invisibility. If the Veil has a Penetration of over 20, it stays up; if its Penetration is 20 or less, the Aegis will dispel the Veil when Bob crosses the threshold.
Oh you did the exercice I provided before for Alice.
We agree for this.
We could also ask ourselve: if Alice cast a group invisibility spell on herself and Bob. For herself she has so much multipliers that her penetration is ridiculously high. For Bob she has 15. When Bob enters the aegis, does the invisibility on Alice stops ("spell dispelled" at group target)? I'd say yes because the spell is group and if it was PeVi'd Id say yes too even if Bob was in Thebes tribunal and Alice in Rome tribunal.
When Alice tries to enter? same discussion as for the PeCo version : the fact that the penetration of the spell on herself is very high is not interesting for the aegis penetration so the 15 penetration (as for Bob) is checked against aegis: spell dispelled on both of them.
Other scenario as Alice: during winter solstice day, you turn a "magical item of doom"TM into a snow flake for sun duration and make it fall above the covenant. The penetration of your transformation into snowflake is 22 (due to another charged item for example so that you know it very precisely) because you studied their aegis with little pink dots to estimate the required penetration.
They renew their aegis during the day and you watch them do so. Now "magical item of doom" is not a foreign item because: ". The first is items that were within the boundary at the time the Aegis was raised. Unlike creatures with Might, non-sentient items become native to the Aegis simply by being present. "
When the aegis is cast, you wait the night, gets your item back (turned backinto item in the snow, but only at night, when everyone is asleep and nobody is looking into the snow that strange wand) with a little effort (happily you saw exactly where you made fall and have some time), and you proceed with your plan against target magus.
Same story, but this time, while they renew their aegis, you follow them turned into a bug (better: as a bug, you lands on the shoulder of the non talisman robe of caster). You are now a participant magus since: "Anyone who participated in the ritual to cast the Aegis is a native of that Aegis. " and nothing requires that the caster agrees that you participate nor is a wizard's communion spell required to cast an aegis.
