Xehanort Report IV

And when Kingdom Hearts is complete, it is said the one who opens its door will bring about the creation of the Next World. Such a feat is above any human. Or, to put it a different way: whoever opens that door will be reborn as something far greater thanhuman.
Light and darkness are two sides of the same coin; without darkness, there is no light. The Keyblade wielders' great war over Kingdom Hearts was fought by defenders of the light, servants of darkness, those who sought to reconcile the two, and those motivated by nothing more than lust for power. A whole spectrum of thought was swept into the conflict, and the worlds that did not go to war found the war brought to them. In the end, the whole World was cast into darkness.
That was how the ancient Keyblade War drew to a close. To this day, no one has ever managed to open the door to Kingdom Hearts.
Some time later, the little light that remained in the hearts of the few gave rise to the World we know today: one made of many worlds behind just as many walls, so that the Keyblade War would not be repeated.