After Twilight and her friends blast Nightmare Moon with the Elements, releasing Luna, they assume that was the last time she would ever surface.

But did it truly work? Did they kill what can not be killed? Cleanse that of which can not be removed? What if she was never truly gone, just striped of her host and left with little power?
Surly such a thing could never have happened, after all they were the good guys and the good guys always come out on top.

Right?

They should have known not to mess with the Dark Alicorn, for the consequences for such a act would be worse then death. After all, You can't fight that which is not real.