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.

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.

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 his pupil Ventus is of no use to him as a new vessel, Master 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 Ventus and his dark half clash once both are of equal strength.

Although the χ-blade is created with their reunion, the forced fusion is incomplete, and the weapon becomes unstable during the fight with Aqua and Mickey Mouse. Vanitas attempts to complete the fusion within their combined heart, but Ventus sacrifices himself to defeat Vanitas, and the χ-blade explodes in the physical world, while Ventus falls into a coma.

Between Kingdom Hearts and Kingdom Hearts II
However, Master Xehanort had anticipated the failure of his attempt to create the χ-blade.

After Terra-Xehanort splits himself into a Heartless and a Nobody, his Heartless, Ansem, manuiplates  Maleficent into capturing  the Princesses of Heart to gathering the seven hearts of light while his Nobody half Xemnas founded 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. 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 explaining to them Master Xehanort's past. Later, after his revival, Master Xehanort reveals to 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 ruined the initial plan, Xehanort accounted for interference and arranged for Ansem to go back in time and share his time-travelling ability with Young Xehanort to gather twelve incarnations of himself from across time to form a new Organization XIII. After failing to "recruit" Sora, 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".