Not much time, so quick thought.
One of the things my character over in Andorra is thinking about, is how to replicate, as much as possible, an Aegis within a Structure.
How would this go?
- 4 ReVi wards (1 per realm) to keep supernatural beings outside. Must penetrate.
- 4 PeVi variation on Impede the intermitent interloper (p109 of Transforming Mythic Europe), but using the "any magical effect" guideline. This would block outside effects coming into the zone with a level under half (base + 4 mags), without any need to penetrate, and effects cast inside if this penetrates.
- 4 Perdo Vim effect to reduce casting totals by half (base level + 2 mags), one per realm. Needs to penetrate
And that's it.
If you wanted to add a dispelling effect, you'd need to add 4 PeVi effect, working on supernatural workings within the zone. This would need to penetrate if the spell is under MR (like, under the parma)
Main difference from the Aegis, at a glance:
- Much, much less effective
- Does not discriminate between friends and foes
- Requires Perdo. In fact, this is much more Perdo than Rego.
IMO, these differences showcase easily the Aegis as a great and powerful breakthrough, difficult to replicate even with centuries of hermetic progress.
I don't think it needs more advantages over "regular" hermetic magic, and I believe that having it otherwise conform to it would make things tidier and more credible.
Under these, this could mean the aegis:
- Needs to penetrate to keep beings with MR out, if their might is under the aegis
- Effects coming from the outside to the inside, under the level of the aegis, are blocked, period
- The aegis reduces your casting total by half its level. Needs to penetrate.
- This is an expansion, but not unlogical: You can't cast anything inside if its level is under half the level of the aegis. This needs to penetrate, though
- The aegis dispels effects crossing its borders. This needs to penetrate.
Yes, it means that if a being is powerful enough, it can mostly shrug of the Aegis.
This is a feature: Having a great Dragon, or the Queen of Summer, walk through your Aegis and do as they please is, IMO, very thematic, and a good reason not to piss them off.
OOTH, if it penetrates, it is a much more effective defence, since it suppresses supernatural effects, including skinchanger
Likewise, it emphasizes the importance of Penetration, and thus, of Day of Communion. IMO, this is a plus: I want my magi to all join their forces to cast the Aegis.
As an aside, I find it interesting that "replicating" the Aegis ended us using Perdo Vim a lot more than Rego Vim 