(Okay, look, I know this is the 3rd rewrite, but bare with me. This one I actually have planned out.)

Everyone knows about the tale of the mare in the moon. But what they didn't know, was that Nightmare Moon wasn't trapped up there alone. Around that time, a hero appeared from out of knowhere, holding down Nightmare Moon so Celestia could use the elements on her. However, this caused the hero to be sent to the moon as well. A thousand years later, and both return. But when Nightmare Moon is defeated... things proceeded to get worse. Like, meta humans popping up in season 1 of the Flash, worse.