A few years ago there was a D&D campaign setting called Midnight, which was set in a fantasy world after the Dark Lord had won and all were enslaved to his will. It was an interesting setting, but I never got to play or run it, partly because as a gaming group we're not that fond of D&D. However, I did think that it might work well with Ars Magica.
Though it started with the players (newly out of apprenticeship) hiding in the cellars of their covenant as it was destroyed by Demons, a few re-workings of the story made me decide that it would be better to start in the prologue, at a time before the rise of the Shadow, in order to show what happened, rather than just tell it as back story.
This is where this campaign begins, in a version of Mythic Europe not too dissimilar to canon. At some point though there will be an upheaval (though it wouldn't be the first time that a campaign of mine has finished before the prologue has ended), so players should be aware not to build characters that won't fit well into a more turbulent (and possibly mobile) setting than you might normally expect.
As far as their characters are concerned though, The Midnight Saga does not threaten any big changes or deviations from the standard Ars Magica canon. It begins at Cruentis Petram, in the Stonehenge Tribunal in 1221.
Some useful game aids.
The saga is using Ars Magica 5th Edition, with a few minor differences.