With the fall of the Old Guard of the Parisian Rite and the rise of François-Thomas Germain's faction, the conservative aristocrats that formed the Old Guard were weakened and hunted down by Germain's men, using the Revolution and the Reign of Terror to execute them due to their aristocratic blood.
Seeing this, the Crimson Rose united some of the surviving members of the Old Guard, the Comte de Choisy among them, and formed the League. Their main objective was to save Templar aristocrats from the guillotine, and they were headquartered at the Hôtel Fieubet. They operated mainly throughout the Marais district, shipping the surviving Templars off the Seine to royalist-aligned locations.
Eventually, the group aroused suspicion from the Republican government, which soon sent a group of soldiers, led by Chauvelin, to destroy the League. The Assassin Arno Dorian, not knowing of the League's Templar affiliation and wanting to investigate more about them, saved the League from the attack and subsequently saved de Choisy from the guillotine as well.
However, Arno was informed by the Chevalier d'Éon of the League's true affiliation, which prompted him to go to the docks and kill off the five Templar nobles trying to escape, de Choisy included. Soon after, he confronted the Crimson Rose at the Hôtel Fieubet, confirming his Templar affiliation and royalist beliefs, before killing him.
With their leadership wiped out, the League was likely disbanded.