After being killed by Sidious, Darth Plagueis became an evil Sith spirit in the dark side netherworld and possessed Sidious. When Sidious died in ROTJ, Plagueis had to find a new force sensitive host body to possess. With a new host body and declaring that the rule of two was a failure, Plagueis has been secretly training a small Sith army in the 30-40 years since ROTJ. With their numbers strong the new Sith army reveals themselves in Episode VII. Confused by the new dark side presence and why balance is not brought to the force. Yoda, Obi-Wan, Qui-Gon, and Anakin's force ghosts decide that Luke must search for the ancient prophecy to discover the secret. Luke finds the prophecy, and it is revealed the there is an evil entity unbalancing the force from the dark side netherworld. As long as this evil entity exist there will always be unbalance, the chosen one must journey into the dark side netherworld and destroy the evil entity in order to truly bring balance to the force. By the end of Episode IX, Luke and his fellow Jedi will have destroyed the new Sith army, and Anakin will have destroyed Plagueis and brought balance to the force.