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This article contains plot details and spoilers for a current or future season.
The Yin-Yang Eclipse is a rare event that occurs rarely in Ninjago, every several thousand years, with only two to three recorded appearances. The most recent eclipse occured some time after the Battle of Stiix, on the annual Day of the Departed. It allowed for Yang to open the Rift of Return, which he planned on using to resurrect himself. However, Cole intervened, destroying the Yin Blade which slowly closed the rift. Prior to its dissipation, Cole and Yang's students crossed through the rift, which, in turn, caused them to turn back into humans.
Despite occuring so rarely, the latest eclipse did serve a purpose, by helping Cole to become a human again and indirectly making Yang at peace.
Dr. Saunders of the Ninjago Museum of History chose the holiday and its coincidental eclipse on which to open the Hall of Villainy exhibit, which the Ninja visited before departing to celebrate the holiday. Unfortunately, Sensei Yang chose to take advantage of the event and Cole's growing insecurity with his ghostly form to regain his old weapon, the Yin Blade, and unleash a group of defeated villains to take their vengeance on the Ninja. Yang warned his pawns that their return from the Departed Realm would only last as long as the eclipse unless they dispatched a living victim with their Departed Blades; they thus set out to attack the ninja and their allies. Meanwhile, Yang took advantage of the eclipse's power and his regained blade to open the Rift of Return in hopes of regaining his own humanity.
Yang's pawns-with the exception of a repentant Morro-were defeated and sent back to the Departed Realm, while Morro warned the Ninja of what was taking place. Fortunately, though they were unable to help directly, their arrival gave Cole the strength to fight and defeat Yang and free his students, who passed through the rift and regained their own humanity. Cole then attempted to take Yang through the rift with him in order to restore them both, as over three thousand years would pass before the next Yin-Yang Eclipse happened. However, bound to the Temple of Airjitzu by a curse, Yang chose to allow Cole to return while he remained "master of the house" and restored the temple to its former beautiful condition.