Χ-blade

The Χ-Blade (Χブレード) is a legendary Keyblade introduced in Kingdom Hearts Birth by Sleep. According to the Xehanort Reports, the X-Blade was an alternative method to opening Kingdom Hearts. As written in Report 7, he stated that "unlike the Keyblade, which existed to join Kingdom Hearts, the X-Blade existed alongside Kingdom Hearts.". Presumably, Master Xehanort believed that lust for the power gained from opening Kingdom Hearts and, thus, the urge to create the X-Blade to open Kingdom Hearts was the reason behind the Keyblade War.

Kingdom Hearts Birth by Sleep
Ventus was 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. The X-Blade was finally created when Vanitas fused with Ventus, though both their fusion and the Χ-Blade were incomplete. Ventus fought off Vanitas (wielding the incomplete X-Blade) inside their Awakening, while Aqua and Mickey Mouse fought off Vanitas/Ventus (who wielded a completed X-Blade) in the Keyblade Graveyard. Thanks to the combined efforts of Ventus, Aqua and Mickey, the Χ-Blade was destroyed along with its wielder, Vanitas.

Within his reports, Master Xehanort stated that the X-Blade could be created when a heart of pure light and a heart of pure darkness intersected, both of their powers being equal. With the creation of the X-Blade, Kingdom Hearts, being the heart of all worlds, would form and one would be able to use the X-Blade to open Kingdom Hearts. As a result, Master Xehanort used Ventus as a tool to create the X-Blade as he split his heart into one of pure light, Ventus, and one of pure darkness, Vanitas.

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

It should be noted that the incomplete X-Blade has the Ultima Weapon coloring to red-orange than bluish-yellow, and the blade is slightly covered with an aura of darkness. It also seems that the incomplete one lacks a part of the Ultima Weapon-shape on one side, what makes it look somehow broken or shattered.

The keychain is a black and red heart that is 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.

Trivia

 * The letter "Χ" is the letter chi from the Greek alphabet. This is why the name of the weapon is spelled "X-Blade", but pronounced "Chi-blade", a pun on "Keyblade". In Japanese, the two are pronounced the same way.
 * 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.
 * The symbols that appear when someone is hit by the X-Blade resemble the ones from Way to the Dawn.
 * When Roxas transforms his two Kingdom Keys into the Oathkeeper and Oblivion near the end of Kingdom Hearts 358/2 Days, he crosses them over each other into an X shape, which may or may not be a subtle reference to the X-Blade.
 * While Sora is in his Final Form in Kingdom Hearts II, Sora is floating along with his Oathkeeper and Oblivion keyblades in X position, which may or may not be another subtle reference to the X-Blade.
 * Xemnas's memories of Master Xehanort's fascination with the X-Blade are the reason why all Organization XIII members are named with an anagram of their original names with an "X" added.