Kingdom Hearts Union by Isaac

Prologue

The Keyblade:

A physical manifestation of the power of the heart.
To each and every person there exists within them a heart.
To each and every heart there lies a degree of power.
The Keyblade is the result of the control and release of said power.

The nature of the Keyblade remains an enigma, it holds a power and purpose understood by few;
It's base abilities seem to indicate that the Keyblade is meant to be a weapon to fight for or against the forces of Light and Darkness and/or a tool to open any locked doors or seals—unlocking them as though Keyblades are some sort of master key, compatible with any lock.

But the Keyblade is not only a tool or a weapon, it is an item with a purpose that, though not known, may extend far beyond simply unlocking doors or fighting.

There are four families of Keyblades:
The Keyblade representing all the Keyblades of the Realm of Light - The Kingdom Key
The Keyblade representing all the Keyblades of the Realm of Darkness - The Kingdom Key D
The most common form of Keyblades are referred to as the Keyblades of the Realm of Light and the Keyblades of the Realm of Darkness. These Keyblades are born of common hearts, the Keyblades of the Realm of Light born of hearts belonging to denizens of the Realm of Light and the Keyblades of the Realm of Darkness born of hearts belonging to the Realm of Darkness.

Then there are the Keyblades of Heart.
The requirement for the creation of such Keyblades are the fusing of 7 pure hearts... Though the only attempt made in history to create such a Keyblade consisted of collecting 7 pure hearts of light, the effort fell short at the collection of 6 hearts.

An incomplete Keyblade of Heart depicted here made with only 6 pure hearts.
It has never been denied that collecting 7 hearts of darkness may suffice in the creation of a Keyblade of Heart.

Then there exists a fourth and final category, of which two Keyblades exist; the Keyblades of Legend.
The first would be the X-Blade, the "X" in the name stemming from the letter symbolizing the end.
The Keyblade of the End - The X-Blade

Depicted the way it was during its one time forging in history.
Such a Keyblade is forged only by the interaction between a heart of pure light and a heart of pure darkness and is similar in appearance to a regular Keyblade of any variety but with powers transcending those of any other; the X-Blade is powerful enough to exist alongside the most supreme of all hearts, Kingdom Hearts, as an equal.

Back in ancient times when all the worlds in the universe were not separated by barriers that prevented their interaction, the Keyblade War occurred; born from the desire to claim Kingdom Hearts via use of the X-Blade.

The result of such an event was that countless lives of Keyblade wielders were taken in the attempt to forge the X-Blade. Because of the accumulation of so many Keyblades in one area, and the deaths of so many people with powerful hearts, there was enormous release of energy that consumed the entirety of the Realm of Light in darkness.

From such a catastrophe, only a few fragments of light survived. Such fragments and the people dwelling within them became the worlds they are today with the barriers they have preventing them from communicating with each other in order to prevent a calamity like the Keyblade War from occurring again.
The second Keyblade in such a category would be one that has never shown its mark on the face of history; The Ultima Keyblade.

Conditions for the creation of such a Keyblade are unknown to anyone, to the point that the very existence of such a Keyblade is doubted.

But from what few records have been preserved, the Ultima Keyblade is described as another Keyblade transcendent above all others; it is both the equal of and counterpart to the X-Blade, the Keyblade of the End.

For such a reason, the Ultima Weapon is consequently the Keyblade of the Beginning...

Again, for emphasis:
Little to nothing is known about the Ultima Keyblade, to try and obtain such information would be the same as realizing the creation of the universe.

Our story, however, centers around the reappearance of such a legendary Keyblade,
its destiny as it pertains to a young boy and girl,
and how everything shall connect back to the heart of all hearts:

Kingdom Hearts