I'm a bit curious about how diseases work in ME. It seems pretty straightforward for a group of mages to summon all 36 of the infernal Decani (demons of disease) in turn & just burn them to the ground with Demon's Eternal Oblivion. Presto! No more disease! That could seriously curtail the Black Death.... Is there a demon of aging?
While some diseases are caused by demons, the majority are from more natural causes.
Imbalanced humors and bad air being major causes.
I think the black death raises questions going beyond how the Order of Hermes would influence things. The church has a lot of power, there are miracle workers walking the earth, prayers to saints are effective, true faith can actually move mountains. So - will there be a plague? Is it part of god's plan and in which way? Answering this will determine the role the Divine will play. If it works against the plague, there may not be much left for magi to do. If not, and the Order decides to get involved with the mundane world, they will be able to save many lives, and it will be noticeable that getting help from wizards improves your chances by a lot more than praying - and that will really turn the world upside down.
I personally, unlike most on this thread, don't particularly care about historical accuracy. History is great for inspiration, but so is myth and anachronistic legends, and so is wild fantasy. And I by and large favor a high-Myth bordering on high-fantasy setting and game.
This is a general problem with the power level in the setting. I personally prefer to heavily House Rule, and play with weaker magic (double all spell/effect levels) and higher Magic Resistance. But more specifically, I'd recommend also:
- Demon princes (and archangels) work like Daemons: you can summon an aspect with Theurgic magic, but the original spirit stays in the Infernal Realm.
- Demons, even just regular ones, are eternal spirits. They cannot be destroyed. You can imprison them in a ring or something, and you can drain them of Might temporarily, but in time if freed they will heal back their Might. (The same applies to angels, and ghosts of all sorts.) That's why there are all these demons bound in magical circles about.
If I were the GM when players tried that, after the first three smitings all the rest would turn up at once and get in a pre-emptive strike. Or if I was in a good mood, the Quaesitors would point this danger out and remind them vigorously of the relevant section of the Code.
My feeling is that the Order of Hermes isn't actually powerful enough to destroy an entire domain of Hell. Individual demons, sure, but not rewriting the ecology of ME.
YGMV, of course.
Edit: It's not so much the power level. It's more that if something this straightforward would work, it would have already been done. That clearly hasn't happened, so the question is why not. I'd allow something with this big a result if it was a lot of hard work, cunning and/or story elements.
I like the characters to have agency. If the characters want to change history, go for it.
Also, I find it tedious if the SG gets a history element wrong, the history buff correcting the SG. History as a useful guide, not as a binding story element is, what I prefer.
One of the key differences between Ars Majica and most games, is the characters can shake the world. The order could stop an army, could eliminate most of the ruling class and establish a new world order, and could greatly diminish, or if they wanted to, spend a lot of effort stop the Black Death.
The key limitation in AM is not if you can, but why would you, and what are the consequences.
No, quite the opposite. Somehow, our sagas end up writing the ... secret history of history? For example, in one of our sagas the Mongol invasion was actually caused by the PCs; it was part of a devious plan to take revenge on House Tremere.
One way to think about it is not that history is set in stone and PCs can't do anything about it; rather, that for them history is particularly pliable towards outcomes that are compatible with recorded history. One good way to encourage this type of play is to provide ample Confidence rewards for actions that steer events in such a direction.
The way we tend to think about it is: what would be a cool way to tie such an event to PCs actions? Maybe it was one of their magical botches that caused it. Maybe it was supposed to be a much longer (everlasting?) famine, part of some plan by diabolists, Muspelli, or just an offended faerie princess; and the PC's heroic actions manage to break it after just two years.
So on the topic of steering things.
I ended my last saga with the death of Konrad von Marburg who'd been a recurring villain, he was historically ambushed on 30.07.1233 and killed by knights in the service of the Count von Sayn, as payback for having accused him of witchcraft / sorcery / devil worship (probably as a power-play over the children of Saint Elisabeth von Thuringen). But I made sure that we ran the session on the 30.07 of that year, and the party were present in Mainz for the acquittal of the Count and then had a grog stalk him. The party had a grog go and scout for good ambush spots and found a small thicket of trees which happened to be at the exact spot where he was killed IRL (if it's a good spot for an ambush, why would they not find it), so that party could see the knights go after the Inquisitor and go and fight their Tytallus nemesis (armed with arcane connections to the whole party) and the demon Guazzo, who had been tormenting the ex Misc and goading von Marburg on.
So history happened as recorded, but not all of the supernatural events got recorded...
Beautiful!