The Elements of Harmony aren’t always the only thing standing between Equestria and the darkness. They are simply the preferred option. Princess Luna learns this one day, when she finally summons the courage to ask her sister a question that had been bothering her for months. "But what if Twilight had failed?"

She is shocked that Celestia actually has a Nuclear option - but not nearly as horrified as Twilight and her friends are when they are tricked into triggering it...