When Cole returns to Yang's haunted temple to seek revenge on the former Airjitzu Master who turned him into a ghost, he accidentally uses a powerful Dark Magic blade that unleashes the spectral forms of Ninjago's greatest villains and traps himself in the temple! While Cole battles for his freedom, the Ninja must return their ghostly foes to the Departed Realm before they gain the power to remain in Ninjago forever! (Please note: This episode is 100% canon. You need to watch it after season 6.) (thetvdb.com)