Twilight Sparkle is an ordinary teenager. Intelligent. Social. Normal.
Midnight Sparkle is a monster from some of the darker fairytales. Killer. Psychopath. Demon. They are the same being. Every night, Twilight becomes Midnight and kills someone to take to hell, as penance for her own crime. The good thing is that nobody knows that the fabled demon actually exists. Until the new student arrives.