Last gaming session we wondere why intruding magi do not simply try to dispel the aegis by using unraveling the fabric of Vim. For a level 40 Aegis you need to get a level 30-dice roll unravelling, which is feasible. Perhaps you could argue that you are casting at/in the aegis, requiring +20 on your dice roll. So if you can get a casting total of about 45 (including dice) you can blast away the aegis, or am I completely wrong? I think that total is not too difficult. What is the use of the aegis then, except for repelling magic creatures.
Serfs Parma but how about size modifiers? The Aegis is generally BIG.
Just a thought.
Well the Aegis is a bigger version of the Parma Magica, and the Parma can't be that easily dispelled (otherwise the whole Order wouldn't have been created) and by that logic I would think that Unraveling the Fabric of Vim would not work on the Aegis either.
It is a ritual spell however (as opposed to the Parma which is a ritual, p. 66) and therefore it should be able to be dispelled; were it not for the spell's description which states that it was a major breakthrough incorporating Mercurian rituals and Hermetic theories and caused its inventor, Notatus, to become the second Primus of House Bonisagus (p. 161).
So I would say that a spell that could successfully cancel the Aegis of the Heart would be a breakthrough that might potentially lead to the discovery of a way to cancel the Parma Magica as well.
Which thus serves as a justification for Quesitorial investigations, intrusion, possible Flambeau and/or Tremere and/or Tytalus (or all three of the above if not more) fabricated charges or direct accusations of Pre-emptive censure of such knowledge, likely even Wizard's War with intent to kill the magus who discovered such a thing.
Remember, the Order is a novel but ever unstable continuum of conflicting egos and agendas which makes any perceived threat a potential death sentence. That's more than enough reason why most sensible magi would be disinclined to even try a thing other than for their own experiments on their own aegis or parma.
Well, you could dispel any Aegis by unravel the fabric of Vim, as you could unravel any other Vim-spell. At least IMO.
However, as other wards, the Aegis has to penetrate in order to take effect, thus any Aegis level 40 should have been casted with a total of at least 80! Still not impossible to dispel but you might need several magae joining a Wizards Communion.
I know, many troupes ruled wards do not need to penetrate. Never the less any Convent will use its own Wizard Communion to boost the level of its Aegis. Thus making dispeling the more difficult...
Vim spell guideline state that an BASE individual target is one spell. Aegis of the Heart is one spell, so you don't need a Target: Boundary.
(You could change that, but maintaining coherancy, it could cause problems if you want to dispell a "group" spell, but that affect only one of your grogs.)
An alternative is to set the level of the dispelling spell higher. (just like that, because you think it should ^^ )
Our troupe ruled that generally a ritual (read: ritual spell) would need another ritual to be dispelled.
Primiraly, this is to put some strenght into rituals : otherwise we thought they were too easy to dispell.
If you look into other books in the Ars Magica line, you will exemple of this : this is were the idea comes from. I could not recall just where we found this about now, but this is of secondary importance.
So, in our campaign, since the Aegis is a ritual, it would need another ritual to be dispelled.
Note that a ritual would need a lot of time to be casted: your grogs would probably notice that there is a group of mage casting a ritual just outside the Covenant door... Then warn the mages, who can disrupt the dispelling ritual. Etc...
I really think that the Aegis protect the Aegis spell.
If the dispell occur from outside the Aegis, it need to penetrate the level of the Aegis (as per the spell definition). It works as for the Parmae Magica.
If it is cast from the boundary, you get 50% of the level if the Aegis as a penalty to casting score. This would probably be easier, but you actually need to cast a ritual from within the Aegis!
Casting it from at Sight or Arcane connection is probably easier: but need a high casting total to penetrate !
If a mage can dispell your Covenant Aegis of the Heart, he is either :
1)- Stronger than you: then the problem with Hermetic Laws will probably not be worth what is gained from dispelling the Aegis of the weak covenant. One have to have good reasons to do this.
2)-Of similar power, but formed a group of magus to cast Wizard ommunion for high penetration and casted the dispelling at Range: Arcane Con. Such a mage would need to convince 2 or more magus to play a part in his unlawful scheme. So it's hard to conceal. Will it be worth it ? Maybe, maybe not. This is resolved in play.
Going around dispelling Aegis is probably the best way to get you Wizard Marched, (if you can afford the cost in Vis.)
But yeah, it's possible.
Note also that the Parmae can be dispelled also, see wind of mundane silence. I don't think I would allow "Unravelling the Form of Vim" to dispel the Parmae. I think I would allow it for Aegis, since it is a Vim spell.
I invite you to consider this as a suggestion.
The best way to solve this is the way you see the more fit for your players and campaign.
Hope it helps you.=)
My point of view too. This is just, IMO, the most sensible view
I don't see how a level 60 Perdo Vim spell that dispels an Aegis is anymore a Ritual than a Level 30.
Its the same spell the only difference is the magnitude, and I don't see the justification for making it an Ritual.
As for the actual dispel itself...
I would apply the penalties of casting within the Aegis itself, even if the caster is outside. I look at it as if the Aegis spell is in the center. To get your Perdo Vim to it, you have to 'wade' through its effects.
what if the boundary is drastically changed?
Like with becoming a living ghost and the problem of using mundane objects as boundaries (such as a stone wall or edge of a field) when these are destroyed (wall is carted off to build a barn, field floods and becomes a lake) the area covered is altered.
If the aegis is cast on a wall around the main buildings, then an area effect eartquake type spell to topple the wall should break the boundary.
Save for the fact that any covenant with an ounce of common sense extends the Boundary to the outerside of the outermost perimeter of their defined covenant area (outside of outermost wall, if there are any defence walls). Thus, whatever earthquake you tried to cast would again have to be monstrous in its casting total to penetrate the aegis to have any effect on the walls in question.
[Edited to add:] At any rate, if someone really wanted to harm magi or others within a covenant's aegis, far easier to just send in some mundane assassins to do it the non-magical way, the aegis can't stop that and your magus has plausible deniability if they are caught. lol.
Level > 50 maybe? 
Um duh, but so what? Its greater than 50; SO?
Just because its 50 doesn't make it a ritual...
Ars Magica 5 page 114 third column:
Actually should read Formulaic and spontaneous spells may not have a level greater than 50
I'd like to be able to spont level 50 spells, btw
Maybe when you get that Major Magical Focus in "Casting Spells"
Or the major Virtue "no botches". Then, I can try, and try, and try, until I roll enough "1"... MWAHAHAHAHAH!!!!
And in flies fatigue...
But back to the actual problem: we (I'm in Thijs' group) don't want just any magus to come dispel our Aegis. And we don't want to ruin the game by inventing some nice Aegis-dispelling magic of ourselves either.
Logical interpretation of RAW (and this threat) leads me to this:
Assume a level 40 Aegis
Unraveling the fabric of Vim (on touch range and using 5 as the average die-roll) has to be level 20. The Aegis imposes a -20 to casting, since I'm casting in the Aegis to affect it. => I need a casting total of 40, losing 1 fatigue to kick this to 30, and a formulaic spell of lvl 20 <- very easy for fresh magi.
The Aegis will get stronger rapidly:
lvl Aegis 20 -> Casting Total = 0
lvl 40 -> CT = 30
lvl 60 -> CT = 60
lvl 80 -> CT = 90
It keeps increasing by 1.5 times the increase in Aegis level.
But think of the Vis you need for proper protection! (at least 12/year)
I think the suggested houserule that ritual spells require a ritual dispel to unravel. This also fair towards other Vis-draining magic IMO.
This however raises the question: "can you boost your formulaic, non-ritual, unraveling spell to a ritual if the situation/target demands it, or would you need to reinvent the whole darn thing?"
Not sure I understood most of that, especially the mention of reducing a casting total by 2 mags by spending 1 fatigue, but as for the last question I would suggest that one cannot "boost" a formulaic spell to a ritual spell since they entail two different manners of casting.
When casting a formulaic spell, if your casting total doesn't reach the spells level (by 1 - 10) you still cast it, but lose 1 fatigue.