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.

After seeing that Ventus is of no use to him as his new vessel, Master Xehanort extracting his apprentice's darkness to form Vanitas. From there on, the elderly Keyblade Master formulated a scheme to create the χ-blade from his former and current apprentices fusing back into one once they are of equal strength. Although the χ-blade was finally created with their reunion, Vanitas's forced fusion with Ventus was incomplete and the weapon becomes unstable as the source of the Unversed attempts to complete the fusion to the core to rectify the matter. After Ventus metaphysically defeats Vanitas at the cost of his own well being while Aqua and Mickey Mouse defeat Ventus-Vanitas, the χ-blade explodes.

Between Kingdom Hearts and Kingdom Hearts II
After Terra-Xehanort's splits himself into a Heartless and a Nobody, his Heartless half Ansem manuiplates Maleficent into capturing the Princesses of Heart to gathering the seven hearts of light while his Nobody half Xemnas founds Organization XIII to gather the thirteen hearts of darkness and also basing Master Xehanort's fascination with the χ-blade to name each of the members of the Organization with an anagram of their original name that contains an additional "X" and splitting Xehanort's heart into the bodies of the members.

Kingdom Hearts 3D: Dream Drop Distance
After his revival, Master Xehanort revealed to Mickey and Riku that he (while possessing Terra's body) figured out the true way to create the legendary weapon: thirteen darknesses needed to be gathered to counteract the seven Princesses of Heart and the resulting clash between light and darkness would again spawn the χ-blade but both plots were ultimately foiled by Sora.

Unfortunately, the villain had accounted for this and planned for his Heartless to go back in time and share his time-travelling ability with younger self to gather twelve incarnations of Xehanort from across time to form a new Organization XIII. After failing to "recruit" Sora, Xehanort's Organization leaves with the intention of meeting again in the future to recreate 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".