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This article contains plot details and spoilers for a current or future season.
The Rift of Return is an opening between dimensions that cuts through the fabric of reality. The rift enables someone who has died or is a ghost to cross through and become living again. However, it is very difficult to access the rift, as according to Zane, one must use the Yin Blade during a rare lunar eclipse that occurs every 3,721 years.
After Yang accidentally transformed himself into a ghost, he used the Yin Blade to try and access the Rift of Return to resurrect himself. While opening the rift, Yang fought against Cole before the two made amends and ascended towards the rift together. Despite this, Yang realized someone had to stay in his temple, prompting him to let go of Cole and allow him to cross through the rift alone.
Once Cole and Yang's students crossed through, the Rift of Return closed.
Long ago, Master Yang found the Yin Blade and discovered its abilities to make anyone and anything live forever. When he tried using the blade on himself, he and his students were transformed into ghosts. Many years later, Yang would use the rare Yin-Yang eclipse, along with the reclaimed Yin Blade, to try and open and rift and resurrect himself. By manipulating Cole, Yang lured him to his temple and took the Yin Blade, after which he commenced the process to open the Rift of Return.
As Cole fought against Yang's students, Yang stood on the top of his temple and began spinning the Yin Blade around, directly underneath the lunar eclipse. Slowly, the blade created an opening in the sky, with the rift steadily increasing in size. Once it was fully opened, Yang used Airjitzu to ascend towards it, but was stopped by Cole, who began to fight him.
Yang clashed with Cole on the roof of his temple, demoralizing and overwhelming the Ninja of Earth, who was slowly fading away. Yet with the arrival of his friends, Cole started regaining hope, to which his arms turned physical once more. Cole threw a single punch at Yang, which shattered the Yin Blade - causing the rift to start closing. Cole told Yang's students to cross through the rift, and once they did so they became human again. Yang was devastated after the loss of the blade and his students, saying that he failed them and all he wanted was to become immortal so that no one would forget him. Cole tells Yang he understands his desire, but assures him that he will be remembered as the creator of Airjitzu. Realizing Cole is right, Yang ascends towards the Rift of Return using Airjitzu. However, Yang realizes that due to the temple's curse, someone must stay in it as a ghost, leading Yang to let go of Cole and fall back towards the temple. Cole continues ascending, crossing through the rift as it closes in a brilliant flash of light.
The Ninja gather around outside of Yang's temple, realizing that Cole didn't make it out of the rift in time - but they're relieved to find that not only did he fully cross through back into Ninjago, but he also became a human once more (albeit with a ghostly scar).