OK. That should probably be appended to the House Rules.
Mercurian Caster is awesome, I've always loved it.
The decision to have the aegis need to penetrate is simple, and your character illustrates it perfectly: You're barely 5 years out of gauntlet, yet you'd be able to keep out powerful creatures on your own (30 being in the upper range of published beasties) => This makes supernatural creatures slightly more dangerous for magi, while allowing more stories: The GM doesn't have to resort to Faerie Princes and Dragons if he wants something to sneak into the covenant.
Also, this keeps the rules unified: It's not that aegis having to penetrate is a HR under 5th, but rather the reverse, like for wards.
As it is, if, say, you've got a CT of 35, you've got 2 options:
- Aegis 30, lvl 30 MR and -15 to casting, keep MR 05 and lower outside
- Aegis 20, lvl 20 MR and -10 to casting, keep MR 15 and lower outside.
Both have their advantages, IMO, which is great
My advice? Hum... Probably the best thing would be for you to take the highest-level WC you can, and have Ra'am invent his lvl 20 Aegis while we learn WC on our own. IIRC, we have a tablet for a lvl 40 Aegis, so one we have enough WC, we'll be able to cast it. Even if it has only penetration for, say, Might 20 critters, the other effects will be quite useful.
What do you think of it?
EDIT: Serf's parma: Muto 05 + Vi 08 + Int 03 + MT 03 (You learned it, not invented it, I guess) + Aura 3 = 22. Unless I'm missing something (and, to speak frankly, I guess I am), you can't even learn such a high-level Aegis.
Actually, it is a house rule. Wards were clarified as needing to penetrate, but Aegis isn't a ward, nor was it ever errata'd along with wards.
And as a Mercurian caster, she already has WC at level 30.
The bit you're missing is that Aegis is Rego Vim. It's Wizard's Communion that's Muto (and she doesn't need to meet the requisites for knowing WC since it's free with Mercurian).
The Fixer indicated it was Serf's Parma, it's not a huge consideration to confuse the techniques when there are as many pieces in play as there are. Despite having the Technique wrong, I still don't think you can learn AoH at 30th level. Re (9)+Vi (8) +Mt (3)+Int (3) + (Aura) 3 is 26. Even with inventive genius applying (which would be an SG decision) you only get to 29.
Like I said above, you're looking for a story reason to join the covenant, I don't think you having the Aegis needs to be that story. Being Mercere is sufficient. Heck if the Aegis thing is that important, you know Wizard's Communion because of Mercurian Magic, so if Ra'am can invent the Aegis in a season, and someone learns Wizard's Communion from the lab text then Ra'am, Azura and the other person can combine to cast Aegis, still key to being able to deploy a decent Aegis within a season.
Being able to invent an Aegis is certainly not my entrance into the Saga story, and I'm more than willing to relinquish it to someone else. What it means is I don't get to play Ra'am as soon as I'd hoped. He goes straight into the lab. When I designed Ra'am I thought long and hard about where he is from, that eventually he'd like to resettle somewhere back in the Iberian peninsula, if not Leon, then Castille or some other local kingdom.
As I progressed in developing Ra'am and reviewing the resources of Phoenix I saw that it was very possible for him to contibute an Aegis, early. Perhaps earlier than the SG had really hoped, I am not sure. As it is, I'm not even sure if there is a functional, non booby-trapped, basic lab available (as aplayer). Whether or not the Aegis penetrates is really immaterial to this discussion, though. You have two players in a troupe style of play suggesting that you spend your starting XPs on something else, which is to your advantage. The Fixer caught my DEO spell not being able to penetrate, while also suggesting that it would eventually resolve itself. Based on that recommendation I changed it accordingly, in my case I got 5xp to spend elsewhere.
I fully disclosed what my intentions for my character were, so that player to player we wouldn't have any difficulty.
I wasn't counting Inventive Genius. I'm including Puissant Rego (+3) and the Magic Theory specialty of inventing spells.
Do you mean you don't get to play him as soon as you'd like if he does have to get in the lab to invent an Aegis, or you don't get to play him unless he's contributing the Aegis? I'm confused. What do YOU want me to do with my character? If you'd rather I don't play a Mercere because there's too much overlap with a Bonisagus then just tell me what kind of character to build.
I missed your Puissant Rego, the +3 isn't listed at the Technique, so it is easy to overlook. I now agree you can learn the spell at character creation. Again, it's not really my point, or even Fixer's point. We mentioned that first but both presented a stronger case against your character needing to have Aegis. The House Rule, which (granted) isn't in the house rule sticky topic, is sufficient reason for you to spend the XPs elsewhere.
Yes, if Ra'am does have to get into the lab to invent an Aegis, I, as a player, don't really get to play him for a while. That's something as a player I can deal with, as it makes sense to the story.
As to your character, it is your character. Have an Aegis spell, don't have an Aegis spell. I'm not concerned with overlap with a Bonisagus by any means. I am saying that two players of the troupe thought you could spend the 30 xp better elsewhere. And again, Ra'am invents an Aegis, you know Wizard's Communion, some other player learns WC, and we cast the Aegis and it will have a very decent Penetration, since Ra'am's CS is 31. Your CS, is about 25. In my mind, it's a win-win, but I might be biased.
Wholehearteadly agrees with Johnathan
With all due respect:
In 5th Ed, all non-indirect spells affecting someone need to penetrate. This is the general rule, and, as such, it isn't restated in every spell description, such as Pilum of Flame, Call to Slumber or Circular Ward against Demons.
It stands to reason that, should an exception exist to that general rule, it'd be stated clearly, as is the case for indirect ReTe spells such as Invisible Sling of Vilano. You'll find no such rule for the Aegis. Thus, why should it be an exception to the general rule, when it isn't stated to be one? I mean, where do you see written "Aegis doesn't need to penetrate"?
Now, used to previous editions, some people (myself included! I remember my surprise when I discovered this) originally didn't require Wards to penetrate, nor Aegis, thinking that was RAW, despite an exception being ever stated.
As the Wards topic brought much, much discussion and controversy, it was clarified in HoH: S ("Like any hermetic spell, these wards must penetrate"). That wasn't enough, some people still thought that they didn't need to penetrate, so that was clarified again in Legends of Hermes. You say it yourself: Clarified, not errata'd 
.
It is the same thing here. The fact that it is by far the most common HR doesn't stop it from being an HR .
You're almost right, though, as, originally, it seems it wasn't supposed to need penetration, but, as it worked fine with it, it wasn't errata'd to be an exception.
Ah, yes! Mercurian get WC equal to their highest ritual. I had forgotten that 
For the record, as I am way too tired to check how that applies or not here, an error I learned the hard way: By Ars5 rules, Inventing isn't learning.
That is, to invent a spell, you start from scratch, gain 1 point per lab total > level, need to accumulate a number of points equal to its level and thus to double its level to learn it in one season. Both Inventive Genius and your specialty applies
To learn a spell, you use a lab text, and need a lab total > its level to learn it in 1 season. Neither Inventive Genius nor your specialty apply.
I think your specialty is a better one than "learning spells" (I took the same), but you might prefer it.
Amusingly, I made the house rule before I fully understood the debate, because I did not want to get into the debate in the first place -- mechanics should take backseat to story, so I asked the troupe which would yield better stories, and went with that option.
I confess that having a PC show up with Aegis memorized would spoil some of my fun, but a Mercurian caster had damn well better know that spell or his mater would wanna know why 
And anyway, I have lots of fun to throw at you guys already.
Has anyone figured out that you've already found some of the Illusory Resources, and the source of the Vim vis?
I am surprised you're all turning inward to solve the Aegis issue. I have a whole world of political machinations waiting for you to try to buy things from other magi, and you're all "I'll just invent it." Gorram self-sufficient bastards 
Oh, and Ra'am, don't worry about being "stuck" in the lab. Gameplay for this campaign seems to encourage lots of "during dinner time" rp.
I'd thought that could be a job for Ettienne given his skillset.
Also consider that we advertise weakness to potential enemies if we ask for a lab text of an Aegis. At least that's where I'm coming from.
All paths are good paths, in my saga 
Azura could always try to swap favours with other Mercurians to WC cast Aegis'es together (communing with non-Mercurian scrubs is such an abhorrent squandering of vis!), which could create all sorts of political subplots.
Speaking of Azura's aegis, I was thinking how her sigil (the colour blue) would manifest in an Aegis. I was thinking it'd be possible that her sigl makes all flowers in the affected area turn blue 
One of her mastery abilities is disguised casting, so she can always fake some other sigil (she'd probably cast an aegis that has an occasional jasmine-scented breeze waft through periodically?); but I like the idea of having one's sigil be unexpectedly inconvenient sometimes 
Not all all, save for Vim... OMFG!!!!!!!!!!!!!!!!!!!!! No freakin'way!!!! 
That's nice, although it shouldn't affect flowers brought in later, which'll make them all the more precious to us: After a while, people will hate blue flowers!!
And the disguised casting is a great idea!
Whistling
Creo Vim Lab total, with Claudia assisting=47 so 5 pawns of Vim...
Ok, it's not so bad...
While Azura is writing/translating her WiCo 30 labtext for Ra'am to use, he can distill 5 pawns of Vim vis from the Aura.
The following season he can learn WiCo 30.
We can do this, people!
You'll be able to recover more than five pawns of Vis vis from under Hiems tower in less than a season, so no worries on that.
And you all have a LOT of plans for Claudia.
Are you kidding? A lab assistant that you don't have to also train arts for? Giving her a Magic Theory book to study from? Please, that' just good lab management!
Yeah, but she'll also try to talk you into experimenting...maybe if you added a reagent here and turned the lens just a shade there, maybe you'd get twenty percent more vis.
Hmm. She'll try.
Personality traits: Scholarly +3, Authoritarian +3, Careful +2. Reinforced with Flawless Magic and Magister in Artibus suggests a rigorous and studious approach to all work, let alone lab work. Ra'am does have a temper, it was shown only once, when he was pushed to the brink and someone interferred with his "life." That would extend to work, now, too. And then he would try to moderate that temper into a lesson, and the lesson would be "boring" (say it like a teenage girl would say awkward when they see someone else do something inappropriate).
I think you're both about 8 inches, with a 1" diameter. Now get back to work.
Not Hiems.
It took a while. I just got this. Well done. 