(Warning. To get full experience, it is recommended to listen to the song "Kryptonite" by 3 Doors Down before reading)
There is are three reasons why Luna became nightmare moon the same year of the fall of the Crystal empire. The first was her envy of her sister. The second was a seed of doubt planted in her mind by Discord. The final of them is the same reason Luna does not have an apprentice. This is the story of the third.

Cover art by the amazingly talented Note Sketch