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.

Before Kingdom Hearts Birth by Sleep
Originally, the χ-blade was the counterpart of Kingdom Hearts. However, the actions of a group who wanted the χ-blade resulted in the legendary Keyblade War that ended when the weapon was splintered into seven fragments of pure light which became the hearts of the seven Princesses of Heart and thirteen shards of darkness. Soon after, the Keyblades were created in the image of the χ-blade, used by those who decided to protect the worlds of light rather than steep them in further darkness as originally intended.

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, when he finds Ventus is able to survive, the elderly Keyblade Master formulated a scheme to create the χ-blade by having Ventus and his dark half reunite once both are of equal strength. Although the χ-blade was finally created with their reunion, the forced fusion was incomplete and the weapon becomes unstable during the fight with Aqua and Mickey Mouse. Vanitas attempts to complete the fusion to the core to rectify the matter, but is metaphysically defeated by Vanitas at the cost of his own well being before the χ-blade explodes in the physical world.

Between Kingdom Hearts and Kingdom Hearts II
However, despite the set back from hasty fusion of Ventus and Vanitas, as well as having trouble controlling his vessel, Master Xehanort had a back-up plan to create the χ-blade. To that end, with Braig's help, Xehanort split his vessel so that his Heartless half "Ansem" can oversee Maleficent gather the Princesses of Heart while his Nobody half Xemnas founded Organization XIII to create thirteen vessels for his heart. Through Xemnas, Master Xehanort's fascination with the χ-blade lead to the naming each of the members of the Organization with an anagram of their original name that contains an additional "X". However, both plans were thwarted by Sora and his friends.

Kingdom Hearts 3D: Dream Drop Distance
Before sending Sora and Riku undergo the Mark of Mastery Exam to use the power of the Keyblade properly, Yen Sid reveals the history of the χ-blade while explaining to them who Master Xehanort is. 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 the seven Princesses of Heart. Through Sora ruined the initial plan, Xehanort accounted for interference and planned for "Ansem" to go back in time and share his time-travelling ability with Young Xehanort 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".