Wandering Tribes
Not much is known about the Bromor tribes, especially not their origin. What little records are left, all are hearsay from neighbouring countries that were written down later. It does not help that part of their history has been actively removed from old libraries, so only little knowledge of them remains.
What we do know or suspect, is that the Bromor tribes were all wandering tribes that would follow the seasons and the animals within their lands. Parts of the year they would hunt, at other times they would forage. While their winters were generally mild, they were well prepared whenever a bad year would strike them. Due to the rough terrain, other countries cared not for these lands.
The tribes did not wage war against each other, but at the same time they originally did not work together. But some people would not be able to follow their tribe due to injuries, age, or other circumstances. While the tribes would still occasionally provide them with resources, these people clustered together in small villages to be safer in number against the occasional Beast.
Cooperation
Each tribe had a dedicated group of fighters, which would both hunt and be ready to fight Beasts. If a tribe discovered a threat of Beasts towards a village, some of their fighters would be sent out to aid the village's defenses. Stories told later, overheard and documented by travelling merchants, talk about a few occasions where multiple tribes ended up fighting against Beasts at the same village. This showed them the difference in their fighting styles.
Intrigued by the differences, the fighters would take some time to learn from each other. The villages became a sharing point of tools, weapons, and experience. As an era arose where Beasts became more numerous, the tribes agreed to share part of their fighters with those that had been left behind. These fighters would stay for a year at a time, before returning to their tribes.
Then a time came where a stampede of Beasts endangered not a village, but a tribe. And from multiple villages, a unified horde of fighters set out and saved the tribe. It was then that their tribe leaders decided to formally work together.
Coalition
The stampede uncovered a weakness in the tribal system, where a single tribe could not handle a large attack. But with tribes on the move, it would be impossible to ask other tribes for aid within a doable timeframe. The solution the tribes arrived at, was expanding the independence of the fighter groups. This became the coalition.
The Bromor Coalition expanded and fortified several villages, adding training grounds and watch towers. While the young and healthy remained on the move, the Coalition forces operated out of bases and went where they were needed. On top of defensive forces, scouts would look for signs of Beasts and lead groups to hunt them down. Materials were brought back and processed, which over time ended up attracting merchants from neighbouring countries.
In the timespan of a single generation, the coalition grew in cohesion. At times they would escort merchants, or hunt dangerous Beasts past country borders. This drew the attention of other countries, where some tentatively decided to cooperate as they saw value in the tribes their approach. But one country disapproved, the Kayonid Empire.
Exodus
The Bromor tribes had never meant offense, they simply sought to eliminate threats to their lands. Vicious predators would claim large areas as their own, not caring about borders, so neither did the hunters. Yet the Kayonid Empire saw these incursions as a threat and an insult. Thanks to the Kayonid Empire's scribes, we know what happened from their perspective.
First, they sent out a small armed delegation to deliver their demands: Full submission. The tribes would have to submit to the command of the empire, send half their fighters to serve in the empire's army, as well as pay taxes. In response, the Coalition representatives said it would take time to contact all tribes. If the empire were to return in two moons time, the Coalition would formally submit.
The Kayonid Emperor did not trust these promises. So he sent a small army in response, which were to arrive a week before the promised date. Their goal was to squash any possible ambush or uprising. Instead, what they found was dismantled villages and not a single trace of the Bromor tribes. Since that day, no formal records exist.
Wayfarer Guild
The Kayonid Empire failed to find any clues as to where the tribes had disappeared to, while other neighbouring countries denied any knowledge. Their formal written records also hold no information. Hiding the truth from the empire's wrath involved destroying so much evidence that even now, with the Kayonid Empire long gone, we still have no formal records of the Bromor fate.
What we can tell, is that nineteen years later these neighbouring countries founded the Wayfarer Guild. With training programs quite alike the old Bromor methods, the guild was allowed to independently train armed forces that were not loyal to any country, instead sharing their knowledge and troops beyond country borders. Over time the Guild grew and spread, until all countries acknowledged its existence as one that stands outside petty warfare.
Frontier
On the Frontier, there's quite a few settlements near that area that can trace back their origins to within a generation of the Bromor Exodus. Some of them are host to several wandering groups, which trade with the settlements whenever their path leads them there. And of course they will send to the settlements both troops to learn, and those no longer able to constantly travel forth. Meanwhile, both wanderer and settlement scouts will search for threats and call for Wayfarers whenever they are in need of larger numbers.
Again, formally nothing is known. If there are still folks remaining that know the truth, they keep the secret well. But now that there no longer is a Kayonid Empire to hunt for those that tricked it, we can formally express that it appears the Bromor Tribes were not just the inspiration of the Wayfarer Guild, but its very predecessors.
I love how they thrive in the danger, not because of the danger itself but because they were able to notice the opportunity of cooperation; yet, in the face of the empire they avoid confrontation with apparent ease.
I didn't want just a sad ending for them, and with their mobility it wasn't hard for them to just up and move away. Would have been hard for them to adjust to new unknown environments, but fortunately they had hidden support from their neighbours.
Too low they build who build beneath the stars - Edward Young