Χ-blade

The χ-blade (χブレード) is a legendary weapon introduced in Kingdom Hearts Birth by Sleep. Despite looking similar to a Keyblade, it is something "altogether different"; rather than being a man-made counterpart to Kingdom Hearts, it coexists with Kingdom Hearts. As such, it is an alternate method to the Keyblade of People's Hearts for opening Kingdom Hearts; instead of opening the Door to Darkness, it summons Kingdom Hearts directly to the wielder. The Keyblade War occurred because of a lust for power that led to the urge to create the χ-blade and open Kingdom Hearts, and Master Xehanort later enacts his plots for the same reason.

Kingdom Hearts Birth by Sleep
Ventus was trained by Master Xehanort so that he may use it to summon Kingdom Hearts to himself, and Xehanort created Vanitas from the darkness in Ventus's heart to achieve this goal. The χ-blade was finally created when Vanitas fused with Ventus, though both their fusion and the χ-blade were incomplete. Ventus fought off Vanitas, who was wielding the incomplete χ-blade, inside their Dive to the Heart, while Aqua and Mickey Mouse fought off Ventus-Vanitas (who wielded a completed χ-blade until Aqua destroys part of it with her Keyblade) 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 χ-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 χ-blade, Kingdom Hearts, being the heart of all worlds, would form and one would be able to use the χ-blade to open Kingdom Hearts. As a result, Master Xehanort used Ventus as a tool to create the χ-blade as he split his heart into one of pure light, Ventus, and one of pure darkness, Vanitas.

Kingdom Hearts II
Xemnas's memories of Master Xehanort's fascination with the χ-blade led him to name each of the members of Organization XIII with an anagram of their original name that contains an additional "X".

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 χ-blade's coloring is more red-orange than bluish-yellow, and the blade is slightly covered with an aura of darkness. It also seems that the incomplete one appears to have chips in the side of the blade, and it lacks a part of the filigree on one side, and part of the Kingdom Key's hilt, that makes it look somehow broken or shattered. 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. When the χ-blade hits a target, the released symbols are gold and dark blue stars, symbolic of light and darkness.

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.

Although χ-blade is pronounced "Keyblade", the letter "χ" is the letter chi from the Greek alphabet. This is why the name of the weapon is spelled "χ-blade", but, as Master Xehanort explains, can be pronounced "Kye-blade". Master Xehanort explains to Ventus, with a visual demonstration of the ancient letter "χ", that he is not referring to a Keyblade that Keyblade Wielders use. He continues to state that the letter itself carries a meaning of "death" and that it "spells endings".