The Realm of Frostfall is one of the regions and Realms in Andoras, located furthest north of the continent beyond the Moonlyt Peaks. Once a mostly-untouched land, the Koshtadic people sailed to Frostfall in the Age of Legends after their former homeland Koshtahdivlann froze over. These people became the Northmen, and while they were aware of their heritage, they did not care about it as much as their Tegrushkin brethren, whom they had a schism with -- the disagreement was over the keeping of old traditions, and while the Tegrushkin argued that they should continue living the Koshtadic way, the Northmen believed that their old ways brought them ruin or disfavor from the gods and decided to settle somewhere where they could find their own lot in life. Later on, the men who would go on to colonize Deimia were actually Tegrushkin who were disillusioned with following the path of tradition, but feeling that they would be killed if they went to Frostfall, instead decided to follow the trade winds south, landing on the uninhabited island of Deimia and forming their own culture there. They were wholly separate from both Tegrush and Frostfall -- in fact, the modern Deimians actually have no idea of their past, opting to have forgotten it altogether. Most Northmen are the same way, but they acknowledge their cultural origins differ from most of the southern Andorasi.
Frostfall was never disunited early on like other kingdoms, as everyone who had sailed there was already a unified people. This was instrumental in beating back the Giant inhabitants of the land, who were the last vestiges of such a civilization, and were in the process of going extinct when the Northmen arrived. The war was bloody, but the Northmen won, and established the Kingdom of Frostfall, which thrived and grew to encompass its modern-day borders. Due to the difficulty of reaching Frostfall, there was never a successful mission of the Five Paragons there, and as such almost everyone still believes in the Old Beliefs, even today. Frostfall was a target like all other kingdoms in the Dragomyr conquest -- House Stenwulf, the reigning kings, were forced to bow to House Dragomyr alongside the Moonlyt Peaks after a crushing defeat in 8 AC, and they had remained loyal vassals ever since, never wavering from the crown's side; however, this was a lost cause, as the Kingdom of Andoras fell in 348 AC with the Civil War's close, and though Frostfall did all it could to keep the peace, it failed. They were one of the first to sign the Treaty of Dragonspire, ratifying that Andoras was in no position to accept more kings and no one could attempt to reinstate the Kingdom of Andoras through conquest.
Frostfall is ruled from the castle of Winterborne. It is the largest region in Andoras, but most of it is not arable due to the land being tundra, and as such the size is only indicative of its vastness, not its power. Winterborne is in the far north and as such relies on imports and supply lines to maintain a comfortable living, whereas most Northmen live in the fertile southern part of Frostfall, which experiences a summer when the ice melts and crops can be grown with relative success.
With the sheer isolation, however, comes exotic flora and fauna that cannot be found anywhere else due to human habitation. Giants are native to Frostfall, and are a rare sight in present day, still biding their time after the war against the Northmen in the Age of Legends. Also of note is that dragons have been reportedly sighted in Frostfall -- the tales seem mostly fictitious, but there may be some truth to the matter, as dragons are known to isolate themselves from human populations, and there is hardly a better place to do so in Andoras than in the cold wastes of Frostfall, where the number of people is nearly outweighed by the sheer square mileage of land size.