The Legion exists to keep the peace between the families. In antique history, the Legion was created following a disastrous war between several large families. The families agreed that such a war should never be allowed to happen again, and they each gave up the majority of their forces to the creation of a single independent force that could ‘police’ the families. There still exists a polite agreement that no family will maintain an army within their steadfasts, though some individuals do command small cadres.
Any disagreements between families are taken to the Legion to decide. The Legion also has the power to intervene should anything major blow up between families, and over time they have also taken on a role as general protector of the families and their steadfasts. Thus, any external threat will be dealt with by the Legion.
“External threats” include dealing with shadow fastnesses in danger of falling. When a normal shadow falls, the resulting chaos becomes part of the nearest debris field (or creates a new one, if the cataclysm was big enough). When a fastness falls, the storms that result can ripple all the way to the Gyre itself in what is called a Rip. Rips can have devastating effects on Horizon itself, causing widespread destruction and death. Avoiding Rips through careful deconstruction is one of the things the Legion is responsible for.
Ultimate control of the Legion rests with the Emperor, a position currently held by Swayvil izikhathi eziyishumi. Beneath her there are officer ranks, including the Legates, Tribunes, and the Centurians. Legionnaires are non-commissioned ranks, led by veterans called Evocati (neither Evocati nor Legionnaires usually possess Vardyger). Between the two groups are the Praetorians, who are elite troops often assigned to protect a Princeps.
Most ranking legionnaires are lords of shadow or stray entitled who don’t wish to participate in the vicious politics of the families. On joining the Legion a person must give up their name and break ties with their family. If they gain rank within the Legion, that rank, and any epithets associated with outstanding service, become their new ‘family’ name. Any person who chooses to leave the Legion may keep their epithets, but not their rank.