Well... It's mainly a question... Exist already a story around the idea of Nightmare or Discord or Sombra wins, Sunset return from the mirror, find Equestria very bad and could not think about return to the human world [Or maybe she is blocked] and decides [Or is forced] to return Equestria to normal?

I suposse someone already think of a idea like this, if so, can someone put a link of some good story?