I agree.
But if you've got thousands of monkey's throwing ballistic knives from Rome to the Levant, one's bound to hit eventually.
Of course, there's no problem once you realise that the game's skirmish-level combat rules are only meant to apply to skirmish-level combats. Precisely what a skirmish level combat is, is (intentionally) ill-defined as it depends on the needs of your troupe/saga/story. However, ballistic knife throwing seems clearly beyond the scope.
Re: 1 and 2: the point is, every magus vs. army situation might be a little different, and there can easily arise one where the magus will not be able to win. Yes, he can create the spell that he needed, and next year win in the fog, and the year after that in the dark, etc. But there are still ways to make it not cut and dried.
Re: 3: invisible characters are not indetectable. They still cast shadows, hence still block the sun.
Re: 4: you're attributing someone else's thought to me. I never said I don't think a character should be able to be Army Killer. My point is that it's still possible to create challenges for a powerful character, if you think about realistic limitations. (We haven't even talked yet about political and social ramifications, where the magus could easily obliterate the army but knows that it would lead to destabilization of his home region, mundane attention towards the Order of Hermes, the death of his brother who's one of the soldiers, a reputation to other magi as an asshole, etc.)
Yep. I didn't forget, I was just embellishing for dramatic effect. :mrgreen:
100 at once? There at least as many possible location as there are archers, if not more. Maybe a single lucky archer will aim straight at you. Even then, a little fastcast ReHe will take care of the anomaly.
If only they could see his shadow to guess his location, but alas it is foggy! 
Indeed. And if the powerful, old magus was wise enough to cast a simple InHe 15 Shriek of the Impending Shafts before entering the battle, he automatically dodges the arrows and doesn't have to make stress rolls for Defense against them at all. 
Cheers
To me this is the relevant bit. Building a magus who can lay waste to a mundane army at 50+ years out of Gauntlet is eminently doable. By 100+ years out of Gauntlet, even magi who aren't combat monsters can probably pull it off with ease.
The question is less 'can you' and more 'should you?'
This, to me, is one of the cool things about Ars Magica. Its not just about the power, its about the ramifications too.
Like, the point in space from where fireballs and pilums of fire originate? No need for a shadow when you give away your position 
Never underestimate the power of the volley of arrows. Note that, againt such a "wall" of arrows, unless you cling stupidly to the mechanics ("I don't care if there are 4 arrows per square feet, the rules say I dodge them all!") Shriek of the Wooden shaft should be all but useless.
Of course, the magus may very well have a ward against wood* (in which case, sooner or later, the soldiers will flee), and even if he hadn't, I still think he'd have a pretty reasonnable chance to roust the army in panic. Funnily enough, I'd say this depends in part on how mythic your game is: In a more realistic saga, the soldiers might flee, thinking they're attacked by the devil, whereas in a saga where mundane armies routinely fight dragons, they'd regroup and know what do to.
Anyway, all that to say that he shouldn't get cocky and assume that invisibility = invulnerability 
Actually there is no point of origin for your fireball in 5th edition. and there is plenty of other attack methods that is not POF/BOAF. If you go for the sneaky assassin I doubt he will be using that kind of spells, and rely in Sight range stuff instead.
IMS we have gone for 2deal with the consequences" idea. Mundane armies end not being problematic if you want to massacre them. Your reputation as an agent of the infernal for such levels of cold blooded mass murder will quickly rise, though (and the interest that demons and angels have in you), so it might be more bothersome than other less messy alternatives. Local resentment mounting against you is not a good thing either. Peasants can be EXTREMELY dangerous if they go against you. Not because they will kill you or anything, but because your life will be more difficult if they do not collaborate with you.
Even if you kill the enemy sinking the invasion fleet, the local merfolk will not be happy. Been there, done that. Not pretty results IMS. The second chance we had about this we decided to SCARE them using a (slightly tricked) dragon. More effective way to break an invasion without real bloodshed. You gain the same reputation of 2do not mess with those crazed dudes" but you imperil your soul way less than just massacring them.
So "deal with the consequences" can be quite funny here. 
Putting sticks in the wheels to keep the peasant rabble a constant treat through the saga is not an approach that we like. As the characters develop into more powerful people, stuff that was dangerous before is no dangerous (physically) anymore. other challenges arise in their place, though.
Cheers,
Xavi
I may be wrong, but IIRC, the text pretty well say that the fireball shoots from your hand.
The fact that, rules-wise, you're creating a fire at voice range doesn't change the fact that the cosmetic bit (which is very real!!!) has it coming from you, just as a Frosty Breath has you breathing mist when you lie, something you can use to fool the one using that on you.
Of course, you can invent a sight range variant of BoaF or PoF that doesn't come from you, something like a pillar of flame that appear on the target, just like you could invent an InMe "lie detector" spell that didn't rely on mist. But the standard spell won't work that way, so you'll have to invent from scratch.
Your magus would be flying, invisible, warded against arrows or using non-standard spells, in which case he should have no problem.
And sure, each time one proposes a situation in which he's not invulnerable, you can say "but he could have prepared for this".
But we come farther and farther from the initial proposition, which was akin to "an invisible flying magus with multicast PoF and BoaF will roust an army any time of the day", to something like "an invisible flying magus with True Sight of the Air, a ward against wood and sight range multicast custom CrIg spells will roust an army any time of the day".
I have no problem with powerful magi being able to roust armies if they're prepared and/or lucky. But I disagree about assuming that, just because they're powerful, everything will always go their way, especially if they didn't prepare for basic possibilities, such as one single archer shooting at them.
This is, IMO, important, not only because of the verisimilitude of the world, but also because it makes a difference between the powerful amateur and the powerful professional, thus rewarding the players who took the time to develop the spells needed for the job.
Wait so. Allowing a flying invisible magus to use SotWS to dodge massed fire is stupidly clinging to the mechanics, but allowing massed archers to shoot at a target hundreds of feet straight up in the air is sensible?
"Quick target that spot the fireballs are coming from"
"Um but sir, we can't..."
"Sure we can, invisible isn't invulnerable men the SG's old school so we can see the point of origin."
"But the arrows still won't..."
"Don't go all rules lawyer on me son. He can't dodge em all it's not realistic!"
"But sir"
"Now look son this is mythic Europe so serfs like you don't talk back now. Just fire!"
Arrows are released, reach their azimuth and return to earth decimating the ranks of the archers own army.
"But...but...they have no maximum range. They should have gone all the way to space."
"Sarge it's Mythic Europe there is no space."
You do realize my original premise call for sight range custom spells. Thus making ward against wood unnecessary unless the SG is a fool who doesn't understand that arrows cant be safely fired straight up at an infinite range. The invisibility wasn't even really for defense. Casting sight spells from hundreds of feet in the air is defense enough in my book. Being invisible just means that the army will be even less likely to realize whats going on and retreat effectively.
I would say that my original premise, (that a flying invisible mage casting mastered sight range area effect spells can defeat almost any mundane army) is almost always is true. Sure if it's foggy he may have trouble, if an experienced Archmage optimized for killing large amounts of mundanes doesn't know or can't spont a relatively simple InAu spell. But really if he doesn't have it he can wait. Armies are really slow and easy to track.
Which get's to my real point. Let the characters succeed when their clearly capable of doing so rather then playing some silly, genital length inspired, I'm smarter then you, pissing contest. It's always easier to criticize then create. Having God (The SG) throw random random roadblocks into his imagined world in situ is not a sign that the player or the character didn't plan completely. It's merely a sign that the SG thinks this should be really really hard and he didn't plan completely. Also probably tiny genitals.
Change the name to Fiery Inferno and have the same effect originate at Voice range without the shooting. Given how the rules where changed for the POF in 5th edition, the current rule has not much sense. If you have 1 dozen people forming a wall side by side, and the target behind them, showing his head only, you can shot the POF at the target without even charring the people in the middle. Regardless of it cosmetically shooting a gush of flames from your hand. Same for Crystal dart, but being a dart is somewhat less blatant when such issues arise.
I note only that your interpretation of an otherwise civil discussion says much more about you than it does about us.
Nope, I'm sorry, I had missed the "sight" bit before PoF and BoaF 
Of course, if you put yourself way above any bow's range, you're safe from mundane arrows
Really, I don't see the problem 
"Cosmetic" effects are pretty real nonetheless, so it's probably a YMMV issue, just like some troupes will say that your clothes will catch fire when struck by a fireball whereas others won't bother. And I think that having them is great for flavor, and shouldn't be ignored.
Even then, if you ignore cosmetic effects and goes purely by dumb, sterile numbers, your basic fire has a size (is it a cubic feet? Larger? I can't remember), so people close to your target's head can very well be burned
I actually agree with you 
We go cosmetic every time. It is just that here we are not discussing what we do IMS, but what the RAW says. And the RAW says that "cosmetic" has no mechanical effect. It is one of the reasons of why I do not like the 5th edition version of POF.
Xavi
??? On the contrary, it does, check "frosty breath of the spoken lie": The mist is explicitely cosmetic, yet you can affect it with Auram, and thus you can fool the caster. If the mist had no mechanical effect, even if supressing it through PeAu, the caster would still know you'd have lied.
Likewise, a BoaF explicitely shoots from your hand, so even if you're invisible, the fireballs will betray your position. Or are we misunderstanding each other about what "mechanical" means?
Legacy spell written all over it.
You can make exactly the same spell without the need for the breath /(you get truth and falsehood directly to your brain) and you cannot be fooled then.
Xavi
Perhaps, but that is one of the main charms* of Ars Magica - it's not all just mechanics. It actually has (had anyway) astrong element of feel.
Yes, but Frosty Breath was designed for use in front of Tribunals.
Mes Culpa.
That did come off a little more flippant then I intended.
My point stands though. It is the SG's job to make the game challenging for the characters. And an SG's narrative control certainly allows him to make any situation a challenge for any character. I think everyone once and awhile forgets that not every situation has to be a challenge. It can also be hard as an SG to admit when what you thought should have been a challenge isn't really. That happens a lot in Ars Magica particularly as Magi grow in power.
It's a fair point. A great story or saga should have the whole spectrum - including the almost impossible task - but also including some "well, gosh, I'm pretty much an expert in that, so ... everyone else take five while I RULE" 
And the storyguide isn't there to "win" (because as you say, that's trivially possible, so what's the point): she's there to bring the world to life.
And communication between the storyguide and the players is always key. Has the player made the magus this capable against armies because he wants harder and harder battle challenges, or because he wants to not bother with us-against-an-army stories?
PS: I've been the storyguide before whose "challenge" was anything but challenging ... to the tune of "Hey, shouldn't my Parma plus my huge Mentem score make it pretty much impossible for any secret faerie to affect me like that?" "Uh ... well ... okay, you guys figure out dinner while I scrap that entire story we just started" 