Before all of this... the world relied on a single source of mana, the giant tree. In the great war, it withered and died due to the wasteful activities of the countries involved.

Mithos and his companions had acquired a great amount of power. The power to change the world. They split the countries that had been at war with one another into two worlds, which went on to exist unaware of one another for thousands of years, exchanging their flow of mana in cycles of regeneration and destruction, while a Chosen One had a role to either continue the line of the Mana Lineage, or to go on the journey of regeneration and ... sacrifice themselves for the sake of the world.

Though in reality ... it was a sacrifice intended to revive his sister all along. And so the cycle continued, with none having the ability to question it.