Battania
One of the Battanian minor factions is the Wolfskins - a society of young warriors who have left their clans to live a life of freedom (and violence) in the woods. So long as they live 'as wolves' - eating no cooked meat, sleeping under no roof, and wearing no woven cloth - they are exempt from the laws of men. The Wolfskins are inspired by the Fianna, a fighting brotherhood who were the stars of a major Irish saga. It treats them as heroes, but they also have a dark side, extorting money from clans who are interested in mundane things like marriage and crops and don't have time to fight all the time. Simon Young, a historian who wrote a brilliantly entertaining reconstruction of "darkest Britain," AD 500, suggests that the Fianna may in reality have given rise to legends of werewolves. The Code of the Wolf however is our own invention.
https://ansionnachfionn.com/seanchas-mythology/na-fianna/
Khuzait Khanate
And for those holdouts who would never submit to a khagan no matter what security or riches he offers, we have a minor faction, the Karakhergits, who keep the old ways.
https://en.wikipedia.org/wiki/Turkic_migration#Rouran_and_Tie-le
Valandia
The Vlandians will have their share of minor factions. As with the Battanians and Khuzaits, these serve to emphasize the origins of the kingdom. For example, players who wish to emulate the adventurers who founded the Vlandian (and Norman) kingdoms can seek their fortunes with the Company of the Golden Boar, mercenaries who are based on medieval outfits like the Catalan or Free Companies.
http://byzantinemilitary.blogspot.com/2014/05/the-catalan-company-byzantine.html
Sturgia
Among the Sturgian minor factions are the Skolderbroda, a mercenary brotherhood based on the (possibly legendary) Jomsvikings of the Baltic. The Finnic tribes meanwhile are represented by the Sons of the Forest, a semi- nomadic clan that practices swidden slash-and-burn agriculture in the depths of the woods.
http://www.ancientpages.com/2016/04/26/mysterious-jomsvikings-fearless-scandinavian-warrior-brotherhood/
http://www.forumbiodiversity.com/showthread.php?t=3273
Aserai
The Aserai, like all Bannerlord cultures, will have minor factions. The Jawwal are Bedouin nomads, like those who plagued caliphs, sultans and kings throughout Islamic history. And though our reference point is late antiquity and the very early medieval era, we've also introduced some institutions that thrived under the caliphs. The Ghilman, a brotherhood of slave-warriors, represent the forerunners to the Mamluks who fought for and later came to dominate the caliphate. Aserai towns meanwhile will be dominated by the back-alley mafias who feature in tales of urban Middle Eastern life, from The Thousand and One Nights to the novels of Naguib Mahfouz.
https://en.wikipedia.org/wiki/Ghazi_(warrior)
http://listverse.com/2018/03/25/10-bloody-facts-about-the-mamluks/
Empire
As in all the realms of Calradia, regional variation throughout the empire will be shown largely through minor factions. Imperial border areas tend to be rougher and more chaotic than the heartland, and in the game are represented by frontiersmen. Though fiercely loyal to the empire, they adopt many of the ways of the warrior-tribes they face, from weaponry to a preference for blood feud over courts of law. The Watchers, in the northern hill country, represent the Akritai, the famous Byzantine borderers of Anatolia. In the south and east, colonies of fugitives and free-spirited wanderers have learned to live and ride like the steppe peoples they fight, forming the Eleftheroi. These are based on the Cossacks -- Third Rome rather than Second, admittedly but probably not much different than those who guarded the Byzantines' Black Sea frontiers. And of course we have a faction based on the Varangians, the fearsome Nord guardsmen of the Emperor, who in the tumult after Arenicos' death have formed a mercenary outfit.
http://byzantinemilitary.blogspot.com/2014/03/akritai-byzantine-border-troops.html
https://en.wikipedia.org/wiki/Khazars
https://www.realmofhistory.com/2016/04/20/10-fascinating-facts-varangian-guard/