My players are currently 20-22 odd sessions into a Hibernian Saga, they're all rather powerful at this point and Templars have started invading Ireland (Hospital was just established) they've expressed interest in destroying Western Society as a whole, what are some things to put in their way to hinder/aid them? Any secret societies with the same goal?
Ars Majica is probably the hardest game to put up meaningful obstacles. The question for magi is rarely "Can I?", but generally "Should I?".
If magi want to overthrow Europe, they can. They could be puppet masters, using "Enslave the Mortal mind". They don't need to be so over the top. Using "Posing the Silent Question" to get an understanding of what competing lords want and then pitting lords against each other. Destabilise the economy by making huge amounts of silver or gold. Create a plague. Magi have absurb levels of power in AM.
The risks to the magi from mundane threats, if they present themselves obviously, is the assassin in the night, poisoned food, the trusted servant/advisor suddenly knifing them, etc. One can't have magic protection up at all times.
An opponent with True Faith is a little bit trickier to deal with, but only a little. Pit of Gaping Earth, Sling of Vilano, or enough penetration can deal with them in a fight anyway. Just like the magi need to worry about the assassins, a True Faith person can be taken out when they are not prepared.
You can throw creaures with Magi Might and Fae in to the mix, which somewhat restricts the magi, but the question is why would they get involved.
The main issue is other magi. Due to the Gift, many magi don't really like people that much. Ruling Europe and/or other people does not appeal to many magi. They just want to be left alone. If a bunch of magi rile up the mundanes in Europe, that will annoy other magi. The other magi are the biggest obstacle.
The PCs encounter an arm of the secret society that successfully created and still maintains the current world\European order. (Beyond humans, there's of course Lizard Men, dragons, and human-sized anthropomorphic rats that report to the demon-deities of the Cathedral of Elich under Stonehenge.)
Nuns keep popping up on the fringes of the PCs activities, and then an outright miracle stymies them. (They need to co-opt the Church? Or God?)
The "court wizard" dudes from Hedge Magic interfere with political maneuvers.
The few but influential economically-imperialist magi of the Rhine and Normandy Tribunals interfere with economic maneuvers.
The PCs launch a plague - or are otherwise mean to the population at large - and draw the ire of a very, very powerful and very, very, very grumpy Tytalus leper-mage.
A Tytalus mage infiltrates the PC's conspiracy. For fun.
A senior Quaesitor shows up. He knows everything! (Oh, no!) And agrees with the PCs! (Whew!) And he more or less takes over. (Uh ...) And it turns out he doesn't quite share the same goals as the PCs.(Oh, no!)
There's an English Hermetic cabal trying to take control of Europe. Hint that "historically" they succeeded. Sassenach!
A queen of the Aos Si shows up and pleads with the PCs to abandon their plans. She claims that one of the Tuatha De Danann, ancient beyond anything, beautiful as spring gentian but twice as bitter, and utterly insane, is behind the PC's plot. To continue is to doom Ireland.
Magical animals show up and want to help. They've been working on their own plan for awhile. Their leader turns out to be the Familiar of an old and perhaps-in-Twilight Bonisagius. The Familiar's plan, though, isn't just the overthrowing of Europe, it's the overthrow of humanity.
It turns out that their idea is far, far from unique. They realize there are at least a dozen Hermetic cabals all trying to do the same thing, or something similar, and just as many fighting them. (This is why we can't have nice things.)
The Templars aren't really there to invade Ireland. They're looking for the PCs. To kill them. (Orders of the Brazen Head?)
The Templars aren't really there to invade Ireland. They're looking for the PCs. To place themselves under the command of the PCs. (Orders of the Brazen Head?)
As the PCs take control of secular leaders they discover that some of them are actually demons. Hell has its own plans for Europe, and they're already well advanced.
As above, but vampires.
As above, but hamsters.
The more control the more that requires control. The PC's plans go very, very well. They soon control a big chunk of Europe. They have to deal with collapsing industries, plagues, invasions, etc, etc, etc. They all now have "Difficult Underlings" in spades. (Plus waves of earthquakes.)
If they succeed, let them see the civilisation they've destroyed get conquered by Arabs or Mongols. If the Mongols conquer western Europe, they may try invading Ireland - Japan was protected by the divine wind, but as far as I can tell Ireland has no equivalent (or if it does, it couldn't stop a bunch of Norman nobles and their Welsh mercenaries).
Indeed. Especially as these magi might charge the PC magi with the high crime of "meddling with mundanes" and - if they don't desist - call for a Wizard's march. Winning the Order over to their agenda might well be the biggest task the PC magi face.
Obstacles:
Other magi. Unless the player magi have a huge amount of political clout, the obstacle is the Order itself. The Order is not especially well organized, but numerous and there are almost always more powerful mages.
If not the Order, probably House Tremere and allies, possibly including House Flambeau (although Flambeau might ally with your magi, depending on the situation). House Tremere will not like the idea of some covenant on the edge of the world upsetting their Europe, and they are a magical military. The world is not for you to conquer, it is for Tremere to rule.
The Holy Roman Empire, the Church, and the suddenly cooperative kingdoms of the Dominion, who are likely to panic, organize, and put up a doomed effort to resist. If the player magi win over Western Europe, they still have the Mongols (highly effective and organized in 13th Century) and the Islamic world (which may unify under a caliphate against such an overwhelming threat).
Faeries will be completely unorganized, but would generally like the chaos, and will support anything that leads to interesting situations. The dark fae like destruction, though, and may offer an alliance.
God. He generally frowns on this sort of thing. A small band of chosen people, possibly including a holy knight, a saint, and a salty crusty mercenary, might find themselves on a collision course with these magi.
Possibly Hell. Demons will have a similar reaction as faeries; they like the chaos such an effort creates, and if the players are destructive, will help. If the players are constructive, the demons are in opposition, but the Infernal forces are not organized and the powerful demons lords are always at each other's throats.
Your players are setting themselves up as villains, so treat them that way.
Well, I think the obvious principle objection would be from mundanes. I would do it in stages.
The first level of opposition would be local lords, that should be fairly easy to deal with. This should slowly build up to royalty or ducal level, perhaps entire armies.
This will draw in the ire of the bishop, that will send in clerics and perhaps a scouting Templar contingent or so on. Then a full crusade. All the preceding should give magi plenty of warning they need to defend themselves from such, and they years to properly do so.
Before the crusade reaches their shores, they will start facing Hermetic opposition. In Tribunal, Certamen, and Wizard's War, and perhaps even clandestine illegal attacks.
The first waves of the crusade will be fairly mundane, but as it progresses more and more Divine fantasy elements would be added. I once designed a paladin-like order of holy knights, coordinating with a similar Priest holy tradition, utilizing a mobile Dominion aura and conducting great Divine Ceremonies, including things like protection from magic, summoning armies of avenging angels, and so on. All based on RoP:TD rules for holy methods and abilities [I despise direct divine intervention, excepting only a subtle one.]
As the fight agains the Divine intensifies, demons would attempt to Corrupt the PCs' plan. Faerie gods, released from dormancy with the fall of the Dominion, will awake strange cults. Perhaps the titans too will stir, with the Muspelli entering the fray. Hedge wizards and angry magi will oppose the PCs directly, including strange mystery cults such as the Dream Witches. And local spirits and beings, such as a dragon, might also oppose them.
All in all, it's entirely possible for the magi to succeed, but they're ultimately fragile and will need to defend against and defeat a wide plethora of enemies, which may prove too difficult.
It depends a lot on if they want to obliterate Europe (most of the mainland reduced to dust and slag with only small populations of workers to meet their basic needs in a few controlled pockets) or if they want to tear it all down to build it back right (a magocracy offering universal women's suffrage, gay marriage, a $15 minimum wage + universal healthcare, etc)
IF THE FORMER
A lot of the above suggestions, especially the holy orders.
As they destabilize various economies in their conquest, a lot of delicate chains of resources break down, leading to mass starvation and riots. This helps their cause until they simply can't easily feed their people, and have to either make deals with entities they ought not talk to, or have to start making supplies with Vis, and suddenly everyone's getting real warped real fast.
In Magi of Hermes, there's a mage who's the fifth generation of seventh son, and there's a prophecy that his seventh son's seventh son will reshape magic as we know it - said guy is on two of seven kids. Someone might see the Hibernian Apocalypse coming down the line and think that prophecy child will be the one to set it all aright, and might get Real Weird about making that kid happen.
Veia is a mad Amazon goddess who knew Bonisagus and still has some of his notebooks. She hates the Order and would gleefully see them obliterated. The Amazons may also be delighted to see the world of men laid low, especially if Veia is in command. She could be an ally or a useful agent.
God, or the Church, or The Devil, or some combination thereof, may decide that it's time to close up shop so Jesus 2.0 is born, but given that He needs to be a fully functional adult to speedrun the Armageddon prophecies, He gets shoved into a faerie regio to age up quick, leading to the faeries meddling in how all of that goes down.
IF THE LATTER
If the players are tearing down The Church and The Divine Right of Kings and all that, the Dominion is 100% going to make a stink, as stated above. Kings also, canonically, have Magic Resistance (Lords of Men, p 41) which might lead to some weirdness where the Pope starts crowning a LOT of Kings of very small nations. Alternatively, we might find out if Presidents or Prime Ministers get a comparable Magic Resistance, and what That Means.
Some of the people closest to the party adopt the ideals of personal freedom, which are being violently enforced by the mages, and decide that the mages themselves have become the sort of tyrants they are opposing, and have to go.
A Folk Witch tradition, for their own altruistic or nefarious purposes, could align with the players and provide them with Stamina Replenishing Potions, allowing them far more capacity to go out and cast big silly world-reshaping spells, or a bunch of mid-level ones. The Folk Witches may have a sort of "Finally, rights for women" mindset, or perhaps A Devil is leading them from behind the scenes, hoping to use the chaos of this radical change for sin.
A nation who's at least a few iterations of conflict away from the main characters sees the writing on the wall. The leaders (of Portugal, perhaps, or Bulgaria) start funneling massive amounts of money into whoever their local traditions are, as official court mages, including some of the (perhaps now fragmented along the status quo/magocracy lines) Order of Hermes.
The Raudskinna Compact, who have a comparatively egalitarian approach to things and broadly want what's best for the world, see the writing on the wall and offer a deal - help us deal with these guys who keep turning into monsters and summoning giants, and we'll gladly aid you in your quest.
The Order's response will be varied, but:
Bonisagus: The Trianoma part of the House will get organized, and probably in opposition unless the ... I need a better name for them than player magi, so Outlaws will do ... Outlaws have considerable political influence. Get used to that phrase.
Tremere: It's our world to rule, not yours.
Flambeau: Some might go Outlaw, but most won't.
Tytalus: Splendid! This half will join you, the other half will go for your throat.
Jerbiton: Die, enemies of culture!
Mercere: We're already organized against you.
Guernicus: We have a one hundred point indictment that will be read at your post-execution Tribunal hearing.
Everyone else will react according to the specifics of the situation. The Criamon might not notice, unless the Outlaws are bringing overt suffering. The Bjornaer may consider this a welcome push-back of civilization. The Verditius will take a while to be concerned, unless commerce is disrupted.
So, for plot hooks, pick any interesting mage to encounter the situation.
Early stages: What in Hermes' thousand names is going on?
Mid stages: Hey, you aren't supposed to do that!
Late stages: We six are only one of a dozen groups Marching on you.
Each mage brings one or two companions, a half-dozen personal grogs, a small collection of magic items, and a unique personality. What else they bring depends on the situation.
In my own game, there are some crazy ideas from time to time. Generally speaking, my take is that this is MYTHIC EUROPE. Players should know that GOD is watching and generally doesn't approve of massive death magic from Wizards, often sending Angels to kill such wizard's while resurrecting all the murdered people and restoring the damage they've done.
This means Wizards have to be practical and surgical about what they're willing to do, in order to avoid God's attention.