The first people to live in the area of the Great Lake were native people of Qaiyore, maintaining a primitive life style.
During the Avarean southern migration in the 4th millenium BE, three tribes didn't go with the rest of the Avaereans across the desert, but parted and went to live by the Great Lake. These tribes were the ancestors of the Beladne. These were the Mantoleyn, the Tanatheyn and the Lachelemeyn.
Developing on the same speed as the other Avareans, by 3000 BE each tribe established itself as a seperate city state, each at the mouth of one of the great rivers. The Mantoleyn settled in the northwest, the Tanatheyn in the northeast and the Lachelemeyn in the southeast.
Because of their higher culture, the Celtotheyn, as the Avaereans called themselves, assimilated most other people between the mountains and the desert. The constant threat of invasion from the desert helped a lot in bringing the natives and new comers together. By 2800 BE there were no natives and Celtotheyn, just Sentoleyn (people of the rivers).
The Sentoleyn lived without disturbance from the outside world for hundreds of years. They united into one kingdom, Rainrin, maintaining a relatively peaceful existence. This came to an end when the aggressive Avaerans came in 621 BE. Seeing Avaeran echos in Rainrin's religion and language they decided to annex the kingdom. The Rainrin, being a somewhat xenophobic people, were't very willing subjects of Avaerand, so the Avaerans conquered them.
The Avaerean conquest lasted 150 years. In 471 BE, Mirish forces, exploring the land, arrived in Rainrin. The two expansionist empires, Avaerand and Mir, started a localized confliect over Rainrin, destroying the land in the process. The war in Rainrin, a relatively small matter for the great empires, since it was on the fringe of both, escalated and eventually led to the terrible cataclysm of 427 BE. In the wake of the destruction of Avaerand, Mir lost interest in the now near desolate land of Rainrin and turned its attention to the more prosperous areas of Qaiyore still not under its control. The land of the Great Lake is nearly forgotten by the Mirish Sorcerers.
But the people remained, and to the people there came a savior. Adanin, a great wizard of Avaerand, was disillusioned with his people. Seeing in the Avaerand of before the rule of Alatta, he saught out among the various colonies of Avarean descent, and chose the people of the Great Lake as isolated and impressionable enough to be perfect for his plan - the creation of a perfect Avaerand state.
So in the year 426 BE, Adanin arrived in the valley of the Great Lake. Masking his identity using magic, he presented himself as Raenath, the Lakeson, the Great Lake embodied in a human being, a personification of the source of life of the people around the Great Lake.
Raenath, formerly Adanin, gathered a group of the brightest people he could find and taught them the magical skills of Avaerand, masking them as the mystical teachings of the Great Lake. Using this group of people, trained magicians in a land reduced to primitive culture, he quickly took over the whole area. Creating a new religion based on blind worship of himself and his followers and giving new hope to the people, he did not meet much resistance.
Raenath formed the structure of the kingdom which survives till this very day. The land is ruled by an aristocracy of wizards. The commoners are led to believe that the wizards are actually priest of the Great Lake, imbued with great power by it and led by an avatar of the Great Lake, a descendant of Raenath.
Because Adanin believed Avaerand was destroyed partly because of its expansionist nature, so he instilled a xenophobic policy. The Beladne, as Raenath named the people between the mountains and the desert, are not to go beyond the Mountains to the north and the Desert to the south.