I've trawled back through two or three years of posts but I couldn't find an answer to this that absolutely clarified it for my (perhaps befuddled) brain. Apologies if I am, in fact, asking something that has been clarified many times over.
Starting from the basis that "Aegis needs to penetrate" (as I'm quite happy to accept the firm statement in Through the Aegis) and an assumption that the covenant has an Aegis lvl 20 that was cast with 12 penetration:
An enemy magus outside the Aegis casts a lvl 10 spell with 20 penetration at the covenant. How is the 12 penetration of the Aegis applied to this attempt?
An enemy maga is inside the Aegis and wants to cast a spell. I understand that she "... must subtract half the level of the Aegis from all their casting totals." How does the 12 penetration of the Aegis affect this calculation?
I suspect I will have to ask follow-up questions!
Thanks in advance for your patience
Penetration applies to people/creatures, not spells.
callen did a big thread about the Aegis of the Hearth. I think he details it here, but I haven't read through the thread in a while
When it's discussed about the necessity for the Aegis to Penetrate, it means that for it to be effective in its ward functionality, it needs to penetrate. The wards and penetration issue is something that reared its head within 5th Edition shortly after being published. With respect to spells cast from outside and from within the Aegis, those mechanics are described within the text of the Aegis. The power of the Aegis is acting against the spell, or rather the spell must overcome the Aegis for it to be effective. Whether the Aegis actually penetrates the MR of a foreign magus, is, in my opinion immaterial. Magi can enter an Aegis that penetrates their MR, the warding capability only applies to creatures with might.
Apparently I had not edited in the bit from Through the Aegis. I have now.
Look down to the section on Might Beings, the explicit comments on them. Of course, that's really the part you didn't want to read, I guess. You'll note that that is the only section discussing penetration.
This is wonderfully useful - thank you, both. Much makes sense now!
Although, a related question occurs which I think contributed to my misunderstanding:
How does the "Aegis doesn't need to penetrate against an 'enemy' spell" compare to the Muto Vim guideline that "... your Penetration total for the Muto Vim spell must beat the other magus's Penetration total for the spell to be changed, unless you are cooperating" (p. 159, Muto Vim Guidelines)? They are both spells that are affecting other spells, are they not?
Thinking about it, I suppose that the Aegis already exists and it is the 'enemy' spell that is trying to affect/penetrate the Aegis.
Yes, as the Aegis is passive in the sense that it only blocks things that arrive at it. Also, consider PeVi instead of MuVi.
That's brilliant, thank you for the assistance!