(SPOILER WARNING) - - - - - - - - - - - - - - - The Green Ninja Prophecy stated that the Green Ninja would defeat the Dark Lord, who is presumed to be the Overlord. The FSM (First Spinjitzu Master) was able to keep up the battle for years because his light balanced the Overlord's darkness. The reason that the Overlord never truly disappears is because it is an evil spirit that exists simply to balance out the light with darkness. As the Overlord once stated, where there is light, there will always be shadow. If the light is too bright, then there will be no shadow, which is also inbalance along with there being eternal shadow instead. I think that the only way to be rid of the Overlord is for Lloyd to embrace his heritage of Dragon, Human, AND Oni. He is trying to represent the light through being, well, good. I believe that it is the reason that the Overlord still exists. It isn't the power of the Oni that would destroy him, it is the balance between light and shadow being recreated through some other means, which I think could be through Lloyd. (Please note that this is just a theory and is by no means confirmed by the writers of Ninjago! I would love to hear your opinions in the comments.)
