Talk:Twilight Sparkle/@comment-142.68.108.213-20130313000608/@comment-26973802-20130314052719

Discord was imprisoned in stone. Nightmare Moon was Luna corrupted (or her dark side possessing her). The evil of Nightmare Moon was pretty much destroyed by the Elements of Harmony.

Still no word on how she returns in the comics (if you choose to go by them as canon), but I'm guessing either the other villain of the arc resurrects the evil from Tartarus or turns the new victim (I'm assuming Celestia, but there's been other theories on who it will be) into a clone.