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
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 equal to the other. To that end, after seeing that Ventus is of no use to him as a vessel and extracting his darkness to produce Vanitas, Xehanort formulated a scheme to create the χ-blade from Ventus and his counterpart fusing back into one once they are of equal strength. However, though the χ-blade was finally created with their reunion, Vanitas's fusion with Ventus was incomplete. As a result, the weapon becomes unstable as Vanitas attempts to complete the fusion to the core to rectify the matter. After Aqua and Mickey Mouse defeat Ventus-Vanitas in the Keyblade Graveyard, as Ventus metaphysically defeats Vanitas at the cost of his well being, the χ-blade explodes with Aqua and the comatose Ventus consumed in the blast.

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".

Kingdom Hearts 3D: Dream Drop Distance
In the aftermath of the failed attempt to create the χ-blade by Ventus and Vanitas' fusion, Master Xehanort, while he was still in Terra's body, figured out the true way to create the χ-blade: thirteen darknesses to counteract the seven Princesses of Heart so that the ensuing battle would create the χ-blade again. To that end, Maleficent is tricked into gathering the Princesses of Heart with Ansem overseeing the process, while Xemnas and Xigbar created Organization XIII with the ulterior motive of splitting Xehanort's heart into their bodies to create thirteen darknesses. However, the plan was foiled by Sora who stopped both Ansem and Organization XIII.

Unfortunately, Xehanort foresaw the event of an interloper and had Ansem give his younger self the power to travel across time to gather twelve members of the new Organization and designated the seven lights to Keyblade Wielders. Realizing that both sides are members short, Master Xehanort and the true Organization XIII left with the intention of meeting again to create the χ-blade.

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".