Reading the most recent "Rivers of London" book by Ben Aaronovitch (who apparently took some inspiration from Ars Magica when setting up his universe), he mentions that when the Ancient Romans couldn't defeat a dangerous spirit (I suspect in an Ars Magic setting it would be a Faerie), they instead turned it into a god and enshrined it safely.
A murderous spirit in ancient London became enshrined as a local aspect of Bacchus, similarly for the local aspect of Isis.
I presume that if this was in an Ars Magica setting, this would mean there was once probably a Mercurian Great Ritual that could bind such a spirit to the authority and/or nature of one of the 'tame' (or at least protective) Roman gods. The Romans did seem to assume that gods of the distant barbarians all corresponded to Roman gods. Maybe it is because that they knew that if they could catch such a 'god', then they could make/force them into an extension of a Roman god/goddess.
Though as as far as I know, there is nothing about this in any version of the Ars Magica rules.
It would be cool if it were though. I would like to include it in my House Rules.
Does anyone see any issues with this interpretation?
I like that. Because they are faeries, the Romans know that they can change them. There is an example in RoP: Faerie of two versions of the same pagan god coming in to conflict with one another (the battle of child eaters on page 67). This take implies that the ancient Romans knew a good deal about faerie nature.
probably changed the glamour of the faerie through the artwork of the shrine, it is forcing the faerie to open itself to such a change (or enticing it) that would be the trick. Enticement is certainly doable in the rules as they stand. A hidden ancient ritual to force a faerie to accept changes, that could be worth pursuing...
Thinking of ancient rituals to do with foreign gods, the Romans did "Evocation" - not in the D&D fireball sense, but in the sense of "calling out" a foreign god to keep it away from a place being sieged or raided, to avoid being cursed. Tremere and Trianoma raided a lot of temples - did they know the secret of Evocation, did they find another way to appease spirits, or did being taught Parma Magica directly by Bonisagus make them so magically resistant they laughed at curses?
Also this would explain why the ancient Babylonians and Persians would capture statues of deities and transport them back to their nation's capital as a sign of conquest - maybe they had a method of pacifying deities, but it requires an Arcane Connection using the most impressive focus of worship of that deity. This would mean any Hermetic Magi trying to adapt it would have to find exactly the right Arcane Connection to use.