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Spoiler ShowThe X-Blade is an artifact that is said to be created during a "high-dimensional clash" between equal and opposing forces of pure light and pure darkness.
It was revealed in DDD that the X-Blade had, in the past, shattered into 20 pieces back during the ancient Keyblade War- seven of light, thirteen of darkness. The seven light shards went on to form the pure hearts of the Princesses of Heart. The darkness shards... yeah, who knows, but that's not important.
Thing is, back in BBS, Xehanort wanted to try out a little "shortcut," as collecting twenty shards would take a lot of time- time that wasn't guaranteed to him if he didn't body-snatch someone soon. So instead of collecting all the pieces right then, he tried to see if a heart of pure light and a heart of pure darkness clashing would do the trick. And it did, sort of... only the pure light component, Ventus, didn't just go along with it, and destroyed himself, the dark heart (Vanitas), and the X-Blade simultaneously.
Long story short, a X-Blade can be forged when equal forces of pure light and darkness clash. 7 +13 = 20 is the proper and most effective way to do this, but it's not the only way, as demonstrated by BBS.