The original Plefir were led by Chieftain Reschag, a decent chieftain who was determined to find a permanent home for his people. For a long time before him the Plefir had been considering abandoning the many small outposts that made up their path around the mountains to settle down in one place where they could begin better exploiting the minerals found by those who manned the outposts closest do the mountains, and Reschag exploited this when it came time to elect a new chief after the death of the old one. The previous Chieftain had begun the process of going along the outposts on one last journey, collecting all the people stationed at those outpousts so that when it came time to finally settle down, the Plefir would be united as one in the new tribe. Reschag promised to not only finish this final journey, but also find the perfect spot for the Plefir to settle down. It's important to note that, while the majority of the Plefir traveled close to the mountains or were stationed near them, therefore being quite used to the cold, there were a few less traveled outposts stationed a bit further from the mountains, with most of the southern ones being completely abandoned.
There was no sign of a struggle, nothing had seemed unusual when the traveling aspect of the tribe had previously been there, except for a few people expressing displeasure with the rest of the tribe, and the only clue was in one rather noticeable and important detail: all the important supplies had gone missing too. While this
may be expanded on more in the future what basically happened was they started disliking the rest of the tribe for not caring about them that much. In fact, their lives were honestly pretty bad compared to the rest of the tribe, having to survive almost completely separate from everywhere else due to the travelers of the tribe almost never traveling to their outposts, something the majority of the other tribes relied on at least in part. This caused a difference between the many southern outposts extending far from the mountain and the outposts actually around the mountain until, eventually, someone elected himself as the southern chieftain and took a large portion of the people manning the southern-most outposts with him further south. No one really noticed due to a lack of easy communication between most of the outposts. As for why these outposts were even created, basically one of the previous chieftains dreamed of Plefir outposts spreading all across the known world, creating a massive tribe that could hold vast amounts of power while not affecting anyone else. He didn't last very long after taxing the shit out of the Plefir to spend all their resources on a bunch of southern outposts. In fact, the really southern-most ones were completely abandoned fairly soon after he died. Anyway, I'm just rambling at this point. Back to the story of the five tribes.
The sudden disappearance of the people of the southern outposts had quite an effect or the rest of the tribe, however. Reschag had been from the southern outposts and knew of the growing divide between them and the rest of the tribe, something he had never really liked. He believed that they deserved to be a part of the new united tribe as much as any Plefir, and was genuinely excited to meet the southern tribes in his journey around the outposts, seeing it as his chance to finally meet the people he had grown up with and welcome them to the northern Plefir with open arms. He was not expecting them to suddenly leave the way they did, and their apparent desertion saddened him for a while, during which time he spent a full day in his tent mourning the desertion of the southern tribes. He soon recovered, however, and the tribe continued their journey across the remaining outposts until, eventually, the journey ended at the northern-most tip of the mountain that would later become their home. Now Reschag needed to fulfill his second promise: Finding the perfect home for the Plefir. This ended up proving more difficult than originally thought due to a number of factors.
The first problem was that Reschag insisted on scouting the area around the mountain himself. He was of the belief that the only way he could get truly accurate information was to scout out the area himself with advisors who knew all about scouting and the properties of the areas he was scouting. Due to being the chieftain, this meant that the entire tribe was basically dragged along usually some distance behind the chieftain while he insisted on saving the majority of their resources for their future permanent home. While this was a small problem at first, it was quickly overshadowed by an entirely different problem. After all, a bunch of beast riding people throwing rocks at you and clubbing you over the head tends to be a bit distracting when you're trying to scout an area out. Now the Plefir were slowed to a crawl, carefully traveling further and sleeping in shifts with at least half of the tribe awake at all times and those who were sleeping being quick to wake up at the slightest noise. Not only that, this meant that gathering resources, hunting, foraging and just general survival suddenly became extremely difficult even for the hardy people of the Plefir. All of a sudden the Plefir were barely moving forward, hungry, thirsty, and absolutely miserable. Poor Reschag who, while he was a decent Chieftain, was simply not ready to so suddenly have the people he controlled be living in miserable conditions due to be hunted by terrifying beast people, did his best to hold the tribe together. Luckily for them, the Plefir were a very logical people at heart and they all understood that if even one of them tried to abandon the tribe for their own safety the whole tribe may very well fall apart. Then, soon before the winter came, it all stopped. The Plefir were not entirely sure as to why it stopped, but they immediately took their chance and began moving south as quick as possible while still doing their best to recover from the terrible conditions they had to deal with for three full seasons. Eventually, they came to an area where two large streams met and formed a raging river. After nothing happened for the next few seasons, Reschag understandably decided that this spot would do just fine.
And so, the Second Tribe was created.