which made the younger Alicorn Princess Luna, grow bitter, eventually refusing to lower the moon to make way for the dawn. Her bitterness “transformed her into a wicked mare of darkness: Nightmare Moon.” The elder sister reluctantly harnessed the power of the Elements of Harmony and “banished her permanently in the moon”, taking responsibility for both sun and moon, maintaining harmony in Equestria.

The name incorporates several plays on words: “mare” as in a female horse, “night mare” as in a mare of the night, and “nightmare” as in frightening dream, derived entirely from the medieval demon which haunts a person’s mind with the use of horrifying visions. In addition, when imprisoned in the moon, Luna appears as a dark area on the moon’s surface; a similar shape in astronomy is called a lunar mare.
The day of Nightmare Moon’s return is also the thousandth day of the Summer Sun Celebration, so Princess Celestia sends Twilight to oversee the celebration’s preparations which are held in Ponyville, and encourages Twilight to “Stop reading those dusty old books” and make some friends. The events of the first and second episodes revolve around Twilight Sparkle and her new friends’ attempt to defeat Nightmare Moon by finding and using the Elements of Harmony, while Nightmare Moon tries to stop them in various ways. At the end of the second episode, Twilight and her friends confront Nightmare Moon and transform her back into Princess Luna, and she emotionally accepts Princess Celestia’s offer to rule by her side again.