My troupe and I are currently designing a new covenant. As sort of semi-beta-quasi-storyguide for the covenant site itself, I plan to take a couple of secret hooks that will be based on the history of the site. I was interested in people's thoughts about sorts of things happened outside the purview of the Romans, or before their arrival.
The site is in an area of Bohemia once inhabited by Celts (they're not just from Ireland any more), never conquered by Rome, and conquered by Slavs during the very early Dark Ages. The Celts kept no written history, and the Romans didn't bring it to this area. Celtic oral traditions will have been eliminated with the Slavic conquest. So there's no record (that I know of; correct me if I'm wrong) to which the canon can refer in deciding about the pre-Slavic history of the area. What's canonical, then? Does a storyguide have a completely free hand: anything he does counts as canonical? Or what?
Also, where can I find out more about the Cult of Mercury? There's quite an extensive history of the Order and the Cult around somewhere, but I've never seen it; I'm limited to certain 5th ed. volumes that are thin on this history.
I'm not very clear on what you're asking. Anything the storyguide decides is cannon for his saga is cannon in his saga, the history books don't have anything to do with it, nor does ArM5 official cannon for that matter. If you decide something about pre-historic Bohemia just make sure it doesn't contradict what the alpha-SG has set up and that he gives you this leeway. Since ArM5 cannon and history don't expand on prehistoric Bohemia at all, I wouldn't see a reason not to give you the leeway to do anything fitting the period in Mythic terms if I were your alpha SG.
As for more material on the Cult of Mercury - it's spread thin, but there is a tiny bit in the description of the "domestication" of the forests of Germany in Guardians of the Forest, plus a smidge of more modern history and current status in True Lineages (under the chapter on House Mercere).
Other than that and core, I don't know of an ArM5 source. You can find a bit in Houses of Hermes IIRC (which you can purchase as a pdf in RPGNow.com) but I wouldn't bother. You won't find much there that you haven't already from the ArM5 books.
The ArM Cult of Mercury is Roman, and described in 2nd edition OoH and nearly 4th edition HoH.
Both books are old, likely unavailable, and I have no time to type chapters of them into a message board.
But the Cult of Mercury has nothing to do with Celts. OK, cultists might have battled the Celts on some occasions, but that's it.
For the relevant early history of Bohemia here the following short overview:
The Celts appear there in the 4th century BC, replaing Illyrians. They themselves are driven out of Bohemia by the Markomanni, a germanic tribe, around the birth of Christ.
On Bohemian territory the Markomanni and Quadi battle Roman armies (with perhaps a few Mercurians in the train) at the time of Marcus Aurelius, staving off Roman conquest of the already planned provinces Marcomannia and Quadia. Roman Legions remain stationed near the Danube at Lauriacum (Lorch today) and Carnuntum (near contemporary Deutsch-Altenburg).
For Celtic culture near the Danube until the 4th century BC, the excavation results of the Heuneburg are a good example.
Does it cost anything? If so, it seems counterproductive to purchase a product that you'll simply be purchasing again in a few months when the 5th Ed version comes out.
While Ars Magica is one of the most technically brilliant games I have seen in my 28 years of roleplaying, I am also new to 5th edition and am disinclined to pick up anything from previous editions, as the "crunch" side of it will likely be invalid. My real gripe with Ars Magica, despite my love of it, is that it assumes a lot of canon knowledge that isn't explicity stated in the current edition's books. One is often expected to simply know what a thing or person is. Makes it not very inviting to noobs, and that's a problem, because otherwise it would draw a lot of people away from d20 and the like.
Given that there is no published ArM5 account of Bohemia beyond that contained within Guardians of the Forest, storyguides have a free hand in deciding "canon" (that is to say, you can choose what you want in your game).
Canon strictly speaking is official ArM5 material published by Atlas Games; any publication for Ars Magica of an earlier edition is now no longer considered canon (although it may be pseudocanonical).
I'd do a search through RedCap for the Cult of Mercury (there is a lot of really creative stuff on the web). There is almost nothing published about the CoM in ArM5, except what is contained in True Lineages, which I highyl recommend.
I think that perhaps I was confusing since I used 'canon' when what I meant was closer to 'paradigm.'
I'm curious how other people solve the problem of coming up with something that both fits the medieval paradigm and also isn't known on the basis of anything knowable to the medievals.
For instance, if someone wanted to have have ancient Germanic hedge magic in their saga, they might look at Tacitus' book on the Germans, and consider his beliefs to be the medieval paradigm. But these Celts aren't known to the medievals (so far as I know), so what fits paradigm?
What's left of them could be faeries - in Bohemia e. g. Rusalkas - now. At least some historical Mitteleuropean faerie stories - like those about Perchta and her following - appear to be shadows of pagan practices.
This would also fit with the ArM5 description of the Faerie Realm (p.187ff).
I don't bother staying true to paradigm at all. I try to stay true to a vision of Mythic europe, a place where legends are alive. I use history, myth, legends, modern philosphy and religion, and off-the-wall ideas to protray supernatural elements in the game. I do not feel restricted to staying in-period or in the area or even close. It is the mythic fairy-tale feel that I want to keep, not the historical veracity.
YMM, of course, V.
Just as an example, I recently told the PCs in my game about Rusalka. She is a singular entity, not a "race" of creatures, and her story was woven with elements of a Chinese movie (The Promise), the Snow Queen Russian legend, and is in general probably fitting to the Novgorod tribunal more than to their area (western Pomerania). I couldn't care less that the story is not historically appropriate for the time and place, it was fun so I used it.