I am planning a covenant that will be based in a massive cave system. Obviously if I want to cover a lot of ground I might need to add a size req to an aegis. But should an aegis be able to cover a cave system? Where exactly is the boundary? I'm tempted to just hand wave it in game but wanted to game it out here. Could an aegis cover a cave system and if so what would count as the boundary? Would you have to install obvious barriers/markers at the extremity of where the aegis could reach, like doors in tunnels?
I would say that if the area of the cave is appropriate that it forms a very natural if irregularly shaped boundary.
Walking it could be a pain, and remember that it requires a breakthrough to add a size level to the aegis since it uses guidelines not really part of hermeticism (though developing alternative Aegises to integrate the principle could produce interesting outcomes)
Assuming that you want the Aegis to cover less than the entire natural cave system (i.e. just a part of it) then I would yes, you would need to do something to mark which parts of the cave system are part of the boundary and which ones are not. Doors and walls are an obvious candidate, but markerstones, fetishes, or whatever could also work. As long as it marks the border in some social and mystical sense.
Do you think so?
I have always interpreted Size to be an exception similar to how the level of the Aegis is also an exception to the "no difference in parameters without a breakthrough" rule. In fact I would go so far as to say that canon has my back here as there are lots of published covenants that dont fit within the standard size of an Aegis. Notably Durenmar which has an entire valley covered by an Aegis.
No, it doesn't. See errata.
The errata needs errata, indicating the phrase needs to be added to the second to the last sentance, since adding it to the penultimate sentence as described results in "Inventing a version with different parameters, however, would require a similar breakthrough and years of research, sizes, or both."
Back to topic, does the cave system have only a single entrance?
If so, an AotH around the entrance might topologically cover the entire cave system, or not depending on House Rules. Take into account what could happen if something tunnels into the cave system.
"Second to last" is exactly what "penultimate" means.
What you quote would be from the last, or ultimate, sentence.
So no, the errata does not need an errata.
I'd certainly house rule (if it came up) that AotH doesn't need original research to include size reqs since the base aegis is tiny.
Specifically for the cave system in question, its part of the covenants virtue/hooks that the cave system is "Vast and labyrinthine" It has multiple entrances, both inside the covenant valley and without, although all entrances are high up in the mountains and hard to find/get to. This will of course result in "Plot" later (how the hell did that random shepherd end up in our lab area?)
I like the idea of marking the boundaries with mystical markers rather than doors, indeed the PCs might want to upgrade to doors after they find things wandering around down in the depths, but to start with marker stones might be better.
I quite like the idea that instead of walking the boundary of the effect, the caster instead walks to each marker in turn and touches it to define the boundary.