And most folks have mentioned Against the Dark, RoP:I, and A&A. So I'll chime in with the following commentary:
Yep, that's way most folks introduce Ars Magica to people - without the magic system. There's enough going on in Ars, just from a historical-mixed-with-fantasy perspective, that taking a couple of sessions to introduce both the world, the general game mechanics, as well as the playstyle (grog/Companion/Magi) is usually considered a wise move. Note that the combat system is...OK. 5th edition has improved it to the point where it's not BAD....but it's not the selling point of the game. It's relatively abstract, so don't get too bogged down in flanking or positioning or 10' movement squares. (Not that that sort of thing isn't fun - just that it isn't AM's strength.) Mainly its a game that has a workable combat system that is designed to let magi be awesome in spellcasting.
Against the Dark: the Transylvanian Tribunal, is the current "dark" sourcebook - but it starts right up in the front with "Transylvania isn't any more spooky than the rest of Mythic Europe - that's an artifact of Bram Stoker. But it's a good place to put the "dark and spooky stuff", so we put it in here." Which, again - not a bad thing, but something to be aware of.
Demons are a bit tricky to play in Ars - due to their nature, they are at the same time very smart and powerful, yet very stupid (from a mortal's perspective). The main issue is that they can't comprehend or act on virtue - which includes things like "maintain a plan in the face of opposition", and "display caution" or "resist their impulses". While they can temporarily "fake it" (by spending Willpower/Confidence), ultimately this is what usually causes their downfall.
Recommendation: like mortals, demons gain back Willpower/Confidence when being true to their Nature. Therefore, when taking a demon as a villain - assume that they spent Confidence to set up their plan. Which includes an understanding of their own weakness, and an ability to learn from past mistakes, and controlling their innate overconfidence. Also assume their strategic plan includes time and places for reflection (ie, to recover Confidence, and to spend that Confidence in order to critique the current state of their plan, and change it accordingly.)
Because once they set up their plan, they won't want to change it: so make sure that the plan includes "times and places to reflect and possibly change." Most plans of this type will probably include "Corrupt a mortal and work through them, because I understand that I cannot comprehend Faith/Hope/Love - but a mortal can." This also means that the demon won't interact much with the mortal world, as the plan it sets up assumes that it won't be able to control its impulses (lack of Temperance).
With that in mind: the plans they come up with will actually be really good (strategically), if somewhat limited by their Nature. Such plans can be broken by PC's, but only if the PC can force the demon to react in a tactical sense - when they don't necessarily have confidence to spend to control their own depravity. When the demon is acting according to its nature, it's actually quite vulnerable - a show of force will drive it off (lack of Hope), and its plans will quickly fall to ruin.
Also note that "Demon's Eternal Oblivion" is the "kill a demon" spell, and it's commonly known and relatively easy to cast. So if a known demon is within shouting distance of a magi, there's a good chance the demon will get shanked. Assuming the demon has spent Confidence to acknowledge its own weakness, it will have plans to address this.