The guideline is actually in HP p.111, confirmed to work on other realms by the "Ars Magica Supplemental Spell Guidelines" document put out by Atlas. Clarified and Expanded from what was on p. 158.
A Ghost with IM is not a Demon. The SG could decide to give them the same special resistances to detection as Demons have, but that would be their game and not RAW.
Unless their AotH is very weak on penetration or they specifically invite the ghost over the edge, it is not getting into the Covenant. If they do invite it, any of them can change their minds at any time. While that would not eject the ghost, it would greatly reduce what it could do.
Additionally doing something that is completely undetectable and has a good chance to corrupt the Covenant without having discussed it with the players before hand is a sure way to lead to the players becoming upset.
EDIT: My bad, the actual guideline and clarification is on p. 111. What was on p.101 was the hint that they could be used on other realms and that was confirmed by Atlas. Also I wrote Spirit when I intended to write Ghost.
For the guideline, I think you mean Realms of Power: Magic, rather than Hermetic Projects? Hermetic Projects does then have some relevant stuff on ghosts on its page 111, though, for bonus confusion.
I don't think it's as clear cut as it definitely working on infernal ghosts. I agree that there's the implication that it can work in Hermetic Projects (in so much as there is a set of effects to affect a Magical Ghost on HP pg 101, and then pg HP 111 talks about building devices to trap Divine or Infernal Ghosts instead). This is a situation in which it's arguably very much in the ghost's favour to let it work, though, so I wouldn't take it as implying it always would.
I do agree I'd be wary about introducing this in what was meant to be a long-running saga unless I was sure my players would be onboard for this sort of thing (although that wouldn't necessarily need to involve discussing it with them, just general playstyle).
EDIT: Also I agree that the general play style of the group is a perfectly acceptable replacement for actively discussing it with the group. However if something like this has not come up in play before I would still advise caution since all it takes is one player to really not like the idea to potentially ruin the game.
It's confirmed that you can use it to create spells to detect creatures of the infernal realm. It's not confirmed (unless I'm missing something) that such spells will be reliable - the supplement doesn't even mention that it won't work on true demons.
That Demon's (and to a lesser extent Infernal) special resistance thing causes all sorts of confusion. I don't know if I am alone in really disliking it, since it throws a monkey wrench in what is otherwise a very precise crunch system.
"It is base # to detect something, unless it is a Demon (or Infernal) then it is the same base but might not work even if cast successfully (and with enough penetration if required) because reasons"
Edit: I am also a little distracted right now, so please forgive me. It is the final week of the regular and (American) football is on.
One of the mages cast the InVi spell "am I in a magic aura", however demonic BS will trump magic divination, so this is a case where I ruled (without telling the players) that the spell wouldn't cut it. The Grog making 15 on his Per+Detect (un)Holiness was the only way to just get the information (another would be to find the numbers of botch dice that I asked them to roll to be unusually high and to work things backwards from there). The grog had a fair chance, since he was rolling 8+stress die...
They do not have an Aegis yet.
Yes, the major has major flaw reckless. He sacrificed a finger to Radegast when visiting mount Radhost (gaining 1 confidence and 1 warping as part of the weird ceremony). The Ghost was forthright about trying to deal in life with the demon Lord Grimnir, the bringer of disasters, to stop the famine. The Ghost even gave them directions to finding a scroll of Infernal lore hidden in her lab, that contained her notes on said demon lord...
I know my players since 15-20 years, these are not just randos I met online.
This is exactly the kind of punches that they roll with. The players of the other mages immediately started joking about having a haunted Covenant.
They are gradually sinking into demon taint, they didn't do any further checked on the vis they obtained from the Earthshaker Bull, despite all the hints avbout the bull stinking mad anger etc They instead kept the cows which he'd studded, and having obtained a jet black calf, are fattening him up to become the Covenant's stud. The corrupted Te vis has been used to make the "Ring of Avaricious Delving".
It is your game and as long as your players are fine with it, do what ever you want.
I have just seen many AM games get messed up over the years (back to 4th) because of the "Demons are completely immune to normal rules" wonkiness. Then you end up having players casting DEO on everything and you have to have the "Talk".
If Magi have no way to detect Demons themselves, the only safe and sane solution is to put up anti-infernal wards everywhere and cast DEO spells on everything. If you do not, you are just asking to get suckered into corruption.
EDIT: And you expanded your post while I was writing mine. It seems your players are perfectly fine with playing in a demon tainted Covenant.
I updated my previous post with a little more info.
That was exactly why I gave them the grog in the first place.
Overall, I don't plan on demons being a major thing in my game, but a slow burning corruption in the background as the reckless mage keeps sticking his fingers where he should not.
So sidebar conversation has said your players are fine with it. I was just trying to point out that not every player in every group would be. This comes down to knowing your players.
My old playgroup, who I have been friends with for over 30 years, has had all sorts of crazy things happen. Granted we mostly play a troupe style WoD with each of us having multiple characters across the varies lines that don't always get along so well with each other. One of the licks got tainted by demons and the rest of us put him down rather brutally. Not per say because he was tainted by a demon, but because he became a massive security risk for us. The player was a little sad since he liked the character but didn't take it bad. However that group has the advantage of playing together for decades, the same as your current group.
Now if something like that happened in my current AM group, most of the players would be rather upset. We have been playing a few years together, served in the Army together before that, and have burned every infernal thing we have bumped into to oblivion. The primary SG has taken the hint that we do not want to have any Demon corruption in the game and avoids trying any sneaky sneaky slipping it to us.
NP, I downloaded and printed that sucker out right when I found it to bring to my groups next game. It is nice to have all on a single document which tells you what books to find them in.
Edit: And I do actually like the adventure. Dungeon crawls are rare in AM and a well designed one is always welcome.
I'm happy to read that. While I would not want to play a pure dungeon crawl game (not gonna name names), I do think that it offers variety. One big reason I have thrown 2 into the campaign so far, is that that social/politics have taken up significantly more of the campaign's time than I had initially anticipated, and more crucially, than I had told my players in our session zero. So combat focussed scenarios are a way to give some of the limelight to the Mu/Re Co and Cr Ig/Te mages.
I designed this dungeon with their character sheets in front of me (we use google spreadsheets).
The choice of making completely sealed rooms in the beginning was to give the Te mage a chance to spont some Te magic as well as to use his peer through rock spell.
The skeletons are there for the Co mage to animate in case of a fight (which I why I was mentioning awaken the slumbering corpse on the discord)
The choice of making the fallen Covenant partly Re focussed was so that I'd have a reason to drop a top tier Re tractatus on the loot list (for the Co mage, who's had really bad luck experimenting in the lab)
I designed this one with the idea in mind that it can be done all in one go, as opposed to the Drachenfels one, where they clearly hit temporary dead ends which they'd have to research their way through.
Instead, my party decided not to fight the automata (for now) nor to break containment on Eginolf's room, but instead to come back on All Hallow's Eve, to find out what was the source of the noise. So I've spend some time thinking of how to turbo-infernalise the dungeon so that it is both the same, but more so.
My current thought is that in addition to the aura being stronger (which is not normal for magic but is for Infernal) and that the ghosts of Tielo and Hermann, instead of just operating their usual routine, will animate the bodies and attack, leading to fights with janky-out of synch skely-ghosts that inflict both real wounds (with the weapon) and fatigue damage (with the ghostly form).
You also have the option of giving them "corrected" information from the ghost of Maria. She could ether feed them false information that will make things harder on them or decide that their Covenant might make a nice place of power for her moving forward in which she might want them to succeed. Her providing nominally useful information is also an effective way to integrate herself with the Magi. Effectively giving her an 'in' for furthering their corruption and raising her status with Hell.
The Ghost of Eginolf would not have found any new allies, since he is still trapped by wards. But any other ghost that survived (Tielo and Hermann) can. Since the players have opened up access to the Infernal Aura, anything looking for an Infernal Aura to dwell in could have taken up residence. This gives you the option to add in additional 'bad guys' if more power is needed.
Oh yeah, the ghost of Maria is going to be a double edged sword. I will open next session with the player(s) conducting her ritual enterrement. From now on they will have a permanent source of Infernal Lore in the Covenant... as well as advice that is not immediately apparent as terrible. One thing which I did not mention is that one of the mages has "plagued by supernatural entities" and that I have not really played much with it so far, for lack of good inspiration.
There is one canonical Infernal character in the region, since the party has never dealt with him, the Bastard of Pfaltz island. But I don't see him rushing in there. I rather see the evil seeming out of the hole and infecting the region, which also ties in to the historical zealot, Konrad von Marburg, which will allow me to tie the Count Heinrich von Sayn, who was both a friend of Albertus Magnus and accused of organising satanic orgies...
I did not mean having some additional big bad take up residence. I was more implying that minor creatures of Infernal Might would consider it a nice secure location. So you could add some bats with Infernal Might for example. Or anything else you might need to make the fight more memorable.
So I just finished the new session, and i thought you might be interested in the follow up.
The reckless mage buried the Ghost of Maria-Valencia in his lab (with her permission) and hired musicians to follow the ritual burial and finished things off by sacrificing one of the young bulls the Covenant has in their herd. This pleased the Infernal Ghost. The Mage got 1 confidence and 1 warping for carrying out this ritual.
The party went back on all Hallow's eve to the much more haunted Dungeon. They battled the two possessed ghost-skeletons, but decided that they would bring them back to the Covenant and buried them in the same lab, sacrificing a further 2 young bulls in the process (no further confidence or warping was given).
The grog did manage to detect that the aura was infernal, although the party attributes the infernal to the specific night alone, rather than the area being permanently infernal (which is a reasonable guess).
One mage got 1 hit KO'd by the ghost's fatiguing ability, and they brought him out to rest, he was visited by his angel to told him to go finish the job.
The rest of the party spent the worse night of their life waiting in an infernal aura, until the mage came back and they all went to battle the final boss (earning 1 confidence for winning).
It is obvious to everyone that the whole thing was infernally corrupted, and one of the grogs managed to make it to Warping rank 1 (gaining witch marks).
The players have completely figured out that it's infernal, sealed it up as best they can.
But they still discuss the right of access to the ghost and how they might share it's teachings.
Oh, and after sacrificing these bulls, the mage in question wants to sacrifice a ram to Radegast, as the party is down to their studding bull (which itself is born of an infernally tainted bull).
I would have never imagined that the player would not just rebury Maria-Valencia at their covenant but inside their own lab!
Seems like you guys are having a lot of fun though. Really great that you managed to simultaneously pull off a dungeon crawl and the ars magica trope of "lets go back and finish this another time".