Χ-blade

The Χ-Blade (Χブレード) is a legendary Keyblade introduced in the Kingdom Hearts prequel, Kingdom Hearts Birth by Sleep. At the time, it was the only known means of opening the door to Kingdom Hearts. It is unknown if it has the ability to perform other tasks.

Kingdom Hearts Birth by Sleep
Ventus's heart was created from the Χ-Blade, and trained by Master Xehanort so that he may use it to open the door to Kingdom Hearts, creating Vanitas from the darkness in Ventus's heart to achieve this goal. It was finally created when Vanitas merged with Ventus, though both their fusion and the Χ-Blade were incomplete. Aqua fought off the Possessed Ventus with the completed X-Blade. While at the same time, Ventus fought off Vanitas inside the awakening. Vanitas wielded the incomplete X-Blade. Thanks to the combined efforts of Ventus and Aqua, the Χ-Blade was destroyed and Vanitas with it.

Appearance
The Χ-Blade appears to be made up of two crossing Kingdom Keys. It has a filigree that slightly resembles one on the Ultima Weapon, and the blade slightly resembles the Dream Sword.

The keychain is a black and red heart (similar to the Heartless symbol, only without the fleur-de-lis bottom and jagged cross) with two Kingdom Keys crossed over it. The chain starts out as two separate chains that form into one. It should be noted that the Incomplete has the ultima coloring to red-orange than Blueish-Yellow. and the Dream Blade is slightly covered with a darkness aura.

Trivia

 * The letter "Χ" is the letter chi from the Greek alphabet.
 * The keychain's two crossed Kingdom Keys is reminiscent of the symbol of the "Keys to the Kingdom of Heaven". Although it is not seen in this incarnation, it is especially significant that the Kingdom Key has a silver blade and gold hilt, and the Kingdom Key D has a gold blade and silver hilt, matching the keys on the symbol.
 * Theoricly, it may be possible that Sora can wield it, since Ventus darkness is Vanitas and he wield it, but this theory can't be confirmed, as the X-Blade was destroyed.