Χ-blade

The χ-blade 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, which eventually become the seven princesses' hearts, 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.

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, both of equal strength, clashed together.

After seeing that his pupil Ventus would be no use to him as a new vessel, Master Xehanort extracted the darkness from his apprentice's heart, creating Vanitas. Once assured that Ventus would survive, the elderly Keyblade Master planned to create the χ-blade by having Ventus clash with his dark half, once he could match Vanitas in strength.

Xehanort sets his plans in motion, and the χ-blade is created at the Keyblade Graveyard through Ventus-Vanitas' forced union. However, the legendary weapon itself is incomplete. Vanitas attempts to complete the fusion within their combined heart, but Ventus sacrifices himself to stop Vanitas. This causes the χ-blade to become unstable during Ventus-Vanitas' fight with Aqua and Mickey Mouse, and it explodes in the physical world, while Ventus falls into a coma.

Between Kingdom Hearts and Kingdom Hearts II
Anticipating failure for his "reckless" attempt to create the χ-blade, Master Xehanort arranges for his ally, Braig, to assist his next incarnation, Terra-Xehanort.

After their experiments plunge Ansem the Wise's other apprentices into darkness and Terra-Xehanort does the same to Braig, he splits himself into the Heartless Ansem and the Nobody Xemnas. Ansem manipulates Maleficent into capturing the seven Princesses of Heart, while Xemnas founds Organization XIII with the intention of using its members as vessels for Xehanort's heart. Master Xehanort's fascination with the χ-blade influences Xemnas to name each of the members of the Organization with an anagram of their original name with the addition of an "X". However, both plans are 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.

After his revival, Master Xehanort reveals to King Mickey and Riku that the actions of Ansem and Xemnas were to restore the legendary weapon by having the thirteen darknesses that are the vessels of Xehanort's heart clash with seven lights.

Although Sora had ruined the initial plan, Master Xehanort had anticipated interference and arranged for Ansem to go back in time and share his time-travelling ability with Young Xehanort, sending him to gather twelve incarnations of himself from across time to form a second Organization XIII. After failing to "recruit" Sora as the final Organization member, the gathered members return to their respective 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.

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 has chips in the side of the blade, lacks a part of the filigree on one side, and part of the Kingdom Key's hilt is missing, making 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.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.

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".

Notes and references
Χ-blade X-Blade X-Klinge