Here is an attempt at a clear version of the current Aegis of the Hearth. This is supposed to make explicit that it does not only affect magi, and that it affects all supernatural effects coming in, but it is not supposed to change anything. However, the ambiguity of the published version means that it may well change things for individual sagas.
Aegis of the Hearth
R: Touch, D: Year, T: Bound, Ritual
This ritual protects a covenant in a way similar to the way in which a Parma Magica protects a magus. It provides very strong protection against supernatural effects and creatures that originate from outside the boundary of the Aegis, and much weaker and less consistent protection against supernatural effects originating within the Aegis. The ritual also allows certain individuals to be excluded from the effects of the Aegis, so that it does not hinder their supernatural powers at all. These individuals are referred to as "natives" of the Aegis in this description, and other individuals, and the effects they create, are referred to as "foreign".
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. A foreign supernatural creature that is within the Aegis when it is created is not expelled, although it does suffer the limits to its powers described later. If the creature were to leave the Aegis, it would need to have a higher Might than the Aegis, or resist the Aegis, in order to enter again. A spirit with Might that is capable of possessing another creature may enter the Aegis inside that creature, as long as it remains entirely passive. It is aware of what is going on, but cannot act in any way, including to end the possession. If the vessel becomes a native of the Aegis, the possessing spirit also does. The spirit can, however, have its invitation revoked (see later) separately from that of its vessel, if the person revoking the invitation knows that the spirit is there.
Any foreign supernatural effect that originates from outside the Aegis and would affect anything inside the Aegis must Penetrate a Magic Resistance equal to the level of the Aegis of the Hearth to take effect. If it fails to Penetrate, the effect fails completely and harmlessly. This applies to all effects, including Intellego effects that would not normally be blocked by Magic Resistance, and effects that would move something across the boundary of the Aegis.
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.
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. This also applies to the supernatural powers of creatures with Might that have been able to enter the Aegis.
Anyone who participated in the ritual to cast the Aegis is a native of that Aegis. Entirely mundane people can participate in the Aegis ritual. At most covenants, only the magi participate in the ritual, to avoid the risk of making disguised demons into natives of the Aegis, but this is a custom rather than a feature of the ritual.
Any participant who has The Gift and has been opened to the Hermetic Arts can invite other individuals into the Aegis. This involves giving the individual a token that was designated as part of the ritual. Individuals that are invited in this way become natives of the Aegis, but they are not able to invite other people in turn. Anyone who can invite someone into the Aegis can also revoke an invitation, whether issued by themselves or by someone else. They do not need to retrieve the token in order to do this. Invitations and revocations can only be issued to specific individuals — it is not possible to revoke "all invitations", for example. Invitations and revocations are effective even if the participant is mistaken about the identity of the individual. A participant can attempt to revoke an invitation even is they do not know for sure that a particular individual has been invited into the Aegis. If they were, then the invitation is revoked, but if they were not, this obviously has no effect. A creature with Might whose invitation is withdrawn is affected in the same way as a creature with Might who was within the Aegis when it was created. Individuals who participated in the ritual cannot be cast out of the ritual; they are native until the Aegis expires, or is replaced with a new one in which they did not participate.
Enchanted items can also be native to the Aegis. There are three main classes. The first is items that were within the Aegis at the time it was raised. Unlike creatures with Might, non-sentient items become native to the Aegis simply by being present. The second class is items created within the Aegis by someone native to the Aegis. The third is items directly activated by a native of the Aegis. Covenfolk who will use enchanted items may, therefore, participate in the Aegis ritual, but many enchanted items do not need Penetration, and thus can be used within the Aegis without any problems. The set of native items, like the set of native people, resets every time the Aegis is recast. Items, however, cannot be invited in to the Aegis.
The Aegis is often cast on the Winter Solstice, as the Magic aura may be slightly higher, and this gives it a full year of duration. It is common for the covenant to hold a celebration for all the covenfolk, and a full council meeting for the magi, immediately after the Aegis is cast.
Aegis of the Hearth was created by Notatus, the first Primus of House Bonisagus. It was a major breakthrough, incorporating Mercurian rituals as well as Hermetic theory, and was the reason Notatus was chosen to succeed Bonisagus. As a result, the spell is more powerful than it ought to be, and has no Perdo requisite. By 1220, Notatus's discoveries have spread, and any magus in the Order is capable of inventing versions of the Aegis with the above parameters. but different levels or sizes. Inventing a version with different parameters, however, would require at least a minor breakthrough, and years of research.