This is an exercise of Hermetic Alternative History about the Schism War.
The Point of Divergence is the timely arrival of the Ex Misc Primus to the emergency Grand Tribunal at the beginning of the Schism War, when the issue of the Diedne-Tremere infighting is to be decided (Lineages, p. 41). The pack of proxy sigils that the Ex Misc Primus carries throws the decisive vote in the favor of the Diedne, that are absolved of all accuses of diabolism and human sacrifice. The Tremere are harshly punished for having broken the peace of the Order and bound to peace with the Diedne. The Tremere Primus refuses to heed the sentence, accusing the mages that voted for the Diedne of having fallen prey to their "corruption". House Tremere is Cast Out for having started a war against another House and having endangered the Order. House Flambeau, always eager for a good fight, House Guernicus, reluctantly seeing the need to sacrifice a House to save the precarious unity of the Order, and House Tytalus, eager to redeem their disgrace, join the war against the Tremere. House Ex Misc and Merinita support their Diedne allies. House Jerbiton and Mercere secretly provide some covert aid to the Tremere, but refrains from official support. The other Houses mostly stay out, even if several mages join the fight on an individual basis. After much fighting, the order vanquishes and exterminates the Tremere (even there are persisting rumors that pockets of Tremere escaped and took refuge in the most remote corners of the world) through the decisive efforts of the unlikely Diedne-Flambeau alliance (and secretly, through dark Guernicus rituals).
What happens next ?