Season 2, Episode 4: Luna Eclipsed
“Take a note, please. To the princess.”
“My dearest teacher, my continuing studies of pony magic led me to discover that we are on the precipice of disaster.”
“Hold on. Preci… Preci…”
“Uh… Brink? Ugh. That something really bad is about to happen. For you see, the mythical Mare in the Moon is, in fact, Nightmare Moon and she’s about to return to Equestria and bring with her eternal night. Something must be done to make sure this terrible prophecy does not come true! I await your quick response. Your faithful student, Twilight Sparkle.”
Reluctantly, the elder sister harnessed the most powerful magic know to ponydom: the Elements of Harmony. Using the magic of the Elements of Harmony, she defeated her sister and banished her permanently in the moon. The elder sister took on responsibility for both sun and moon, and harmony has been maintained in Equestria for generations since.
The bitterness in the young one’s heart had transformed her into a wicked mare of darkness: Nightmare Moon. She vowed that she would shroud the land in eternal night.