While the majority of the Continent is fertile and able to support agricultural civilizations (by design), there are certain areas near mountains or in locations where the soil isn't that great, where the only source of plants for miles around is grass. Thusly, nomadic tribes focused around herding popped up in these areas, most notably in The Urysas mountain ranges and in northwestern Ravenna. Little is known about them, other than that they caused significant troubles for the civilizations around them. Upon the conquests of The Alammar Empire they weren't given much of a second glance- whatever nomadic tribe attacked the roaming Alammaran armies were quickly routed and sent scurrying with only a single battle.
After the Alammar Empire began the long process of nation-building and solidifying their territories, the mountain tribes and steppe nomads once again became a problem, raiding nearby villages and even sacking entire cities in some cases. The Alammaran army, still experienced and filled with veterans from the initial Alammaran conquests managed to defeat every group they came across, but the problem is that they never stayed down. Despite High King Alfred's best attempts, Alammaran scribes, bureaucrats and soldiers were unable to pacify and catalogue the regions where nomadic and mountainous tribes resided, and after a costly few campaigns trying he gave up, electing to manage his already consolidated territories. Successive reigns of High Kings did the same, until High King Oswald took the throne.
In an audacious move, on the week of celebration that came with the ascension of a new High King and the week of mourning for the death of the old one, a group of mountain peoples raided inward from the Urysas, heading straight for Ajandah. They caught nearly the entire Imperial Army off guard and on break, with the rest of the garrisons caught off guard and under prepared for any fighting. They managed to carve a line straight to the outskirts of the city before being stopped by a joint citizen-militia and Imperial Army force, though only just barely, being one of the few enemies since the Four Kings to nearly directly attack Ajandah.
This was unacceptable to Oswald, who consolidated the Imperial Army and declared the Pacification Wars, a conflict that would long outlast him. Its objective was to completely neutralize the threat of mountainous raiders and nomadic tribes beyond the jurisdiction of the Alammar Empire. It was not very successful, leading the inexperienced army to defeat a few times in the early parts of the war and then causing an expensive goose chase in the latter part of the war that lasted for decades. Still the nomadic tribes persisted, despite the best efforts of the Alammar, who decided to drop it due to the expenses of the war and the strain it was putting on the imperial Treasury. While a significant blow was dealt to their population, the Alammar Empire had still not managed to fully colonize the grasslands, their settlements and forts often failing due to lack of agriculture and an overreliance on imperial supply lines, or falling to nomadic raiders due to their general isolation from the rest of the Empire.
These raids from nomadic groups would add to the pile of straw on the proverbial Alammaran camel's back. When the Splintering broke out, these groups managed to thrive in the chaos, carving out large sections of Ravenna among the chaos and disunity. These barbarian kingdoms and civilizations, without the constant unity produced by the hardship of the nomadic life would grow weaker and more complacent in record time, allowing the Kingdom of Vysokiy to eventually conquer them and the rest of Ravenna. This also wouldn't be the end of the nomadic tribes, they would continue raiding, at least until the Age of Frost, when the Armies of Dread came from northwestern Ravenna and crushed the nomadic tribes up there to a last man.
While mountainous tribes would persist for a little while, the climate changes brought on by the Age of Frost forced them to move further downwards, where they were more easily able to be counterattacked and more vulnerable to pacification, ultimately reducing them to a minor threat.