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.