A century before the Doom of Valyria , the North was undisputedly the strongest of the Sunset Kingdoms , encompassing an area equivalent to the rest of Westeros combined (or so it was claimed , more recent estimates put its actual size at more modest deminisions.
Nevertheless the North was rich in resources and had beautiful cosmopolitan cities that had traders from the iron isles all the way to the distant shores of Yi-Ti and the shadow lands, in addition it was quickly matching the lands of Essos in technological advancements (though they would never come close to rivalling Valyria itself , the dragonlords' secrets of engineering and the forging of Valyrian steel remaining a mystery to all).
The South on the other hand had undergone very little change since the days of the Andal conquest, a land of chivalry, jousts and tournies where knights competed for riches and prestige . More importantly however were the constant bitter wars , primarily between the Stormlands and the Reach with the Lannisters occasionally playing kingmaker sometimes (when they didn't try their own hand at revanchist expansions).
The lowest point in the south's strength came when the Riverlands , which finally started to recover from its decimation at the hands of the Burner was reclaimed by the very same northerners without a fight , the other kingdoms not wanting to risk the Northerners ire so long as they left them be.
This left the heirs of the Builders free reign to expel many Riverlanders (who had remained after the decimation out of a desire to rebuild their homes and having no where else to go in many cases) . Learning from their previous attempts at ruling the lands of Rivers and hills , the Northerners would not let their conquered land be given away by smallfolk who saw them as heretics and so gave the people there a simple choice : either convert to all the old gods or be expelled.
Some chose the former , most "chose" the later (it is now widely accepted that many Northern commanders would depopulate entire villages without ever giving them a choice and while this was not condoned by the Starks , they also did not do much to punish "overeager" soliders. )
This would lead to an influx of refugees to the southern kingdoms, which at first welcomed them warmly as prosecuted followers of the seven but soon the displaced Riverlanders found themselves receiving lukewarm treatment at best , relegated to shanty towns and subject to extremely high taxes (as repayment for their hosts' "generosity" ) and being denied land to settle on. Many Riverlanders would turn to the faith , the masters or join the westerosi mercenary companies ,that desired to profit from the endless conflict .
It was one of these Riverlanders who would change the course of Westerosi history.
Allen the Great or as he was known at the time , Allen the cobbler's son was born in one of the slums surrounding Lannisport, where he would spend the first twelve years of his life helping his father at work until both he and the boy's mother fell ill to a plague of Sleepox that swept across Westeros at the time. Orphaned and without any other relatives in the dangerous underbelly of the golden city , he nevertheless somehow managed to claw his way up until he was working as a servant to house Lannister of Lannisport.
We have little in the way of accurate historical details but we do know that he came to apprentice under the house's maester before being sent to the citadel. Though later writers would document that Allen immediately caught the eye of his lord who saw he was destined for greatness and had him tutored , other sources claim that he was caught neglecting his duties in favour of ears dropping on the maester as he taught the lord's children and rather than being punished the grey beard took him in as a side project, perhaps impressed by the boy's audacity.
No matter the reality of the matter what is known for certain is that Allen would study in the citadel for four years and forged many links in his maester's chain (despite coming to blows very often with his teachers for what they saw as rebelliousness and unorthodox methods). He would eventually leave the citadel and made an attempt at joining the clergy before deciding the life of a mercenary was more appealing to him. As a sellsword he found immense success, joining up with the Star swords company where he would spend the next decade fighting everywhere from Dorne to the Vale , facing all manner of enemies from Ironborn reavers to Stormlander foot soliders and even Northern pikemen on occasion. It was during these years of travelling that he would come to see an opportunity to make his own mark on history.
After rising through the ranks to the point where he commanded the Star Swords , Allen began to dramatically shift the focus of the company . He began massively recruiting smallfolk from all over Westeros , remarkably allowing women to join (something no reputable companies ever did ) and enforcing a strict meritocracy. With his massively expanded army it was obvious that he couldn't pay them all for long but Allen had a solution to that.
Renaming the company into the Sword legion (which at this point numbered a respectable five thousand fighters though most were peasants with not much in the way of training. This doesn't take into account the number of camp followers and auxiliaries) he would take his new legion and begin carving himself an empire in the lands east of the Riverlands , his elite core of hardened mercenaries proving to be more than a match for the armies of petty kings and knights that squabbled in their little corner of Westeros (unclaimed by the North for its distance and only marginally noticed by the Stormlands which eventually found the land more trouble than it was worth to hold ) so none of the kingdoms thought of stopping him but while the lords didn't pay him much heed , he was becoming infamous with the faith , because Allen didn't restrict himself to conquest by blood and steel but by words and faith as well.
As the "legion" became less and less of an actual sellsowrd company and more like a functioning state , Allen began to preach his own views on the Faith of the Seven , views which would be considered heretical anywhere in Westeros save perhaps the more unconventional northern sects. He proclaimed that the faith of the seven had forsaken its followers and became little more than obident sycophants to the nobility while letting the faithful poor suffer , going as far as state that the they twisted and corrupted the original messages of the Seven-who-are-one in order to suit their needs.
At first he was dismissed as nothing more than a madman , there had been plenty like him before but they never amounted to much in the grand scheme of things. However as Allen proved himself to be a strategic genius , steadily absorbing more and more lands to his burgeoning empire (wether by conquest or diplomacy) and as more and more flocked to his banner from across the continent , he became impossible to ignore . Even more so as his teachings of equality among all (be they man or woman, or more dangerously noble and lowborn) began to spread among the smallfolk, causing many small uprisings in the lands of particularly cruel lords.
The upstart conquered turned revolutionary preacher would face many assassination attempts but he always survived to fight another day , much to the dismay of his growing number of enemies.
Part of the reason for Allen's success (aside from his sharp mind and charisma , a fact even his most bitter enemies acknowledged privately) was his willingness to adapt and adopt the tactics and ideas of his enemies as was most evident when he began hiring northern sellswords to train his now numerically impressive but low quality army and mould them into a force similar to the newly formed northern legions . This was tied to his pragmatism as he was notable for keeping discipline in his men for the most part , only allowing merciless looting and sacking when a certain village or castle had proven resisting to his forces, even then he would invest heavily in rebuilding whatever was destroyed, carefully crafting himself the image of a reluctant ruler who was forced into such acts by greedy nobles who would rather see their castles drowned in blood than surrender peacefully (wether he actually believed this or it was merely a clever ploy again we can only speculate).
This all came to a head when the forces of Allen clashed with Northern forces near blackwater bay.