I think one of the first things is to establish how obvious do they want to be? I just can’t see any way to do the large change wanted, a Magiocracy, without being very obvious, so I’m going with that theme.
Your description implies they are of the ends justify the means type. The gift is a big problem, but not insurmountable. If an outside threat is big enough, scared people will line up behind monsters (and the Gift has peasants putting magi in the monster category), if that monster will keep them and their family safe.
- Create a plague. Wander London, perdo corpus a bunch of workers. Aim for food handlers, couriers, etc, to make sure you’ve got a decent disease transmission vector. Aim for sailors and docks workers if you want all of Europe.
Have cities covered by a year long Purification of the Festering Wounds.It would be resource intensive as boundary is a ritual, and it would also warp, but peasants wont know about warping, and even if they did, warping is a small cost to pay to stay alive.
The magi make it known at the start of the plague they will protect cities that ask for their help but only so many. The peasants hear about the rest of England losing 1 in 3 people, but lets say Birmingham and Manchester asked for help, they are losing 1 in 100. The peasants will love the magi. The Lords will be completely dependent on the Magi, as the magi could always take their protection away. While I’ve said make it obvious, clearly not obvious the magi started the plague.
- Engineer a war between feuding lords. You could do mundane scheming, you could literally use enslave the mortal mind, Whatever fits the skill set of the group.
End the war. Again, go with the skill set. Fly over the battlefield. Have the extreme combat mage mastered group Incantation of lightning blast 30 trees and demand the war ends. Enslave the mortal mind, demand the war ends. Be showy. Make it clear the magi have absolute control.
Even a war is small beans in the big picture. I’m not as knowledge with history as others, but the continent spanning war like WWI, WWII, Napoleonic, etc, are a while away. While the 100 year war is big, it’s mostly France and England and that is 100 year away. I think a plague or something similar is the cataclysm to aim for.
- Demand allegiance.Prior to this, visit lords and give a display of power. Literally kill ever lord in England who does not swear allegiance. A Corpus specialist could Seven league stride and Clenching Grasp of the Crushed Heart a lot of targets. Throw in a veil of invisibility for safety. You could split the task between magi. A lord’s bedroom being torched, another mysteriously drowning in his own bed, to increase the terror.
The issue is the kickback. The order will push back. How you run the Divine matters. Does God care if Magi rule? Ars Majica implies angels may intervene in Earthly matters. There are miracles. If a cataclysm gets big enough is God’s hand felt more? The easy answer is no, and I don’t think it is a bad answer, but it needs to be considered, especially if a Demon is at the helm.
Ars, especially magi with 30+ years out of gauntlet, is rarely a question of can I, it is a question of should I? Enough vis, enough time, and the right magi, with boundary rituals you can go crazy. We are often talking level 50+ spells, but a shady cabal, willing to have magi act as lab assistants to buff study totals, making high level spells becomes reasonable.
You could make a flood of near Biblical proportions. You could reproduce the plagues Moses sent to the Pharoah. The options are near endless, only limited by the specific skill sets of the magi.