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, instead of opening the door to an artificial Kingdom Hearts like the Keyblade of heart, it actually materializes the true Kingdom Hearts, a perfect union of all hearts and worlds. The Keyblade War occurred because of the many souls who desired to take the χ-blade and Kingdom Hearts for themselves, and Master Xehanort later enacts his plots with the same goal in mind.

Early history
Originally, the χ-blade was the counterpart of Kingdom Hearts, for which it acted as protector. However, many who sought to take the light of Kingdom Hearts for themselves created "Keyblades" in the image of the χ-blade, and they clashed together in the legendary Keyblade War. As a result, darkness overtook the World, Kingdom Hearts vanished, and the χ-blade was splintered into seven fragments of pure light and thirteen shards of darkness. When the World was revived by the lights within children's hearts, some decided to use the Keyblade to defend that light. The seven fragments of light broken off from the χ-blade eventually become the seven princesses' hearts.

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 with both equal to the other.

After seeing that Ventus is of no use as a new vessel, Xehanort extracted the darkness from his apprentice's heart to form Vanitas. Once assured that Ventus will survive, the elderly Keyblade Master formulated a scheme to create the χ-blade by having his former pupil and current one clash once both are of equal strength.

At the Keyblade Graveyard, Xehanort created a Kingdom Hearts and then the χ-blade was created with Ventus-Vanitas' forced reunion. However, the legendary weapon itself was incomplete. When Vanitas attempts to complete the fusion within their combined heart but Ventus instead sacrifices himself, the χ-blade becomes unstable during Ventus-Vanitas' fight with Aqua and Mickey Mouse then explodes in the physical world while Ventus falls into a coma.

Between Kingdom Hearts and Kingdom Hearts II
Anticipating failure for the "reckless" attempt to create the χ-blade, mercenary Braig was in place to assist Terra-Xehanort. After their experiments plunge Ansem the Wise's other apprentices into darkness and "Xehanort" does the same to Braig, the apprentice splits himself into the Heartless Ansem and the Nobody Xemnas. Each of Xehanort's splintered halves enacted a plan form the true version of the χ-blade: Ansem manipulates Maleficent into capturing the seven Princesses of Heart in order to form the a special Keyblade that will open a Kingdom Hearts within the End of the World while Xemnas and Xigbar founded Organization XIII with the intention of using the thirteen members (which were each given an anagram of their original name with an "X" due to the fascination with the χ-blade ) as vessels for Xehanort's heart using a Kingdom Hearts created in the World That Never Was. However, both plans were thwarted by Sora and his friends.

Kingdom Hearts 3D: Dream Drop Distance
Before sending Sora and Riku to undergo the Mark of Mastery exam, Yen Sid reveals the history of the χ-blade while also explaining Xehanort's past. Despite the initial plan being ruined, this was accounted for as Ansem went back in time and share his time-travelling ability with Young Xehanort to gather twelve incarnations of himself from across time and space to form the True Organization XIII within the Sleeping Worlds. After failing to "recruit" Sora as the thirteenth "member", Xehanort's Organization return to their individual places in time 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".