Yin Blade

The Yin Blade is a sacred and extremely dangerous device used by Master Kodokuna Yang. It resembles an Aeroblade attached to a handle on a chain. While its blades are sharp, it is intended to be used as a mystical tool rather than a weapon, with the ability to open rifts in the fabric of reality and the potential to unleash great devastation if broken.

History
While searching for the key to immortality, Master Yang's studies led him to discovering the Yin Blade. Experimenting with its mystical, life-giving properties, he used it to restore the life of deceased flora, before attempting to use its magic on himself in order to achieve immortality. However, the experiment was flawed, and instead it resulted in him and his students becoming ghosts, trapped between the realms of the living and departed and unable to leave the Temple of Airjitzu. Sometime afterwards, the Yin Blade was discovered and transported to the Ninjago Museum of History, where it was encased in a case of Clearstone and put on display in an exhibit.

Day of the Departed
While exploring the Hall of Villainy exhibit, the Yin Blade was discovered by Cole, who discovered its relation to Yang, even as the Master himself set his plan in motion by goading him into bringing the artifact to him, taking advantage of his panic upon the former's discovery of his fading existence. Terrified and demanding answers, Cole complied, bringing the Yin Blade with him to the Temple of Airjitzu, confronting Yang with the weapon and attempting to attack, only to accidentally shatter a jar containing an exit from the Departed Realm and activating it, freeing the souls of Samukai, Kozu, Cryptor, Chen, Morro, Eyezor, Zugu, and multiple Nindroids and releasing them onto Ninjago.

Triumphant with the presence of the Yin Blade and the souls of past villains to distract the rest of the ninja, Yang proceeded to seize the weapon from Cole with the help of his deceased students and explained his plan to use the eclipse to catalyze the powers of the Yin Blade and cut a rift through reality that would allow him to escape from his state of limbo.

With the Yin Blade in hand, Yang proceeded to go to the roof of his temple and activate the artifact, using its reality-warping powers to open the rift of return. However, Cole, confronting him, attacked Yang, forcing the latter to defend himself with the Yin Blade with success, slicing apart Cole's blade in the process. However, when Cole's support by his friends powered him up, Cole used his charged fists to attack Yang, who attempted to shield himself with the Yin Blade. Unable to withstand the assault, the Yin Blade was shattered by Cole's attack, and with its destruction and the ending Yin-Yang eclipse, caused the rift of return to close as a result.

Powers
A mysterious artifact of unknown origin, the Yin Blade had life-restoring abilities, able to bring deceased flora to life with its rejuvenating abilities. Despite these miraculous powers, however, the Yin Blade was also flawed—when used to grant Yang immortality, the attempt backfired, instead turning him and his students into ghosts trapped between life and death.

Aside from these abilities, its blades also had the ability to tear rifts through fabric of reality, allowing it to create passageways through realms to a limited extent, and could tear apart most weapons with ease as a result of its incredibly sharp blades, rivaled only by the Anacondrai Blades. These abilities, when catalyzed by the magic of the Yin-Yang Eclipse, were potent enough to even create exits from the Departed Realm, a feat even the Realm Crystal is incapable of.

Its reality-splicing abilities also allowed it to curse and confine victims within a certain area, although some, such as Cole, are able to escape the full affects of this if they escape the Yin Blade's area of magic in time. The effects of its curses are practically permanent, able to last hundreds, if not thousands of years and cause affected areas to become linked to other realms and the Ethereal Divide at night.

Naturally, the Yin Blade is fueled by a massive amount of incredibly powerful magic, granting the sacred tool its restorative and reality-splicing abilities; If shattered, the release of so much magic could affect the Sixteen Realms in devastating ways.

LEGO Ninjago: Masters of Spinjitzu

 * "Day of the Departed"