Xehanort's Report III
In ancient times, no such buffers kept the larger World divided; there were no walls of light enclosing each smaller world the way they do today. Nothing prevented one from physically interfering with the next.
At that time, the World was filled with light, and a great many Keyblade wielders. But without buffers, the worlds began to squabble over ownership of the light. You see, they learned of the Keyblade's true use. War broke out as each of the Keyblade wielders struggled to lay claim to an almighty entity known as Kingdom Hearts.
Kingdom Hearts could be described as an aggregate of hearts. Worlds, too, have hearts, just like people; each world's heart is concealed from sight, beyond a hidden door. By gathering all the hearts of all the worlds in a single place, one may complete Kingdom Hearts.
Xehanort's 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 than human.
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.
Xehanort's Report VI
There are three families of Keyblades: the Keyblades of light we wield, Keyblades of darkness, and Keyblades of heart.
The first and second families differ only in origin; Keyblades of darkness are found in the realm of darkness, and are counterparts to the Keyblades we use in the realm of light.
The third type of key, the Keyblade of heart, came into existence when the World was reorganized after the Keyblade War. Without this key, Kingdom Hearts is forever beyond a person's reach. Only by gathering seven hearts of pure light--hearts completely devoid of darkness--may one forge a Keyblade of heart and open the door to Kingdom Hearts.
And, as stated before, opening this door arguably gives that person control over all worlds and all people.
Xehanort's Report VII
I have uncovered the Keyblade's ultimate mystery. You see, besides the three families of Keyblades, there is another "Key Blade." While it may sound the same when spoken, it is notated uniquely: "χ-blade." And make no mistake, while it resembles a normal Keyblade, it is something altogether different.
Keyblades are said to be man-made counterparts to Kingdom Hearts. The χ-blade, however, coexists with Kingdom Hearts.
It is only forged when two hearts of equal power intersect--one heart of pure darkness, one heart of pure light. At the time of its forging, Kingdom Hearts appears. It must be noted, though, that this Kingdom Hearts is special. Unlike the Kingdoms brought about forcibly and artificially through the collection of hearts, THIS Kingdom Hearts is a perfect and complete union of ALL the worlds' hearts. Surely it was over this that the ancient Keyblade War was fought.
If so, the walls that divide the worlds today are of little consequence. With the χ-blade, all their hearts could be instantly reunited--and the Keyblade War, refought.