Princess Luna, known as Nightmare Moon or Night Mare Moon when transformed or under certain other circumstances, is an Alicorn pony, the younger sister of Princess Celestia, and the main antagonist of the season one premiere of My Little Pony Friendship is Magic as Nightmare Moon. She is also the adoptive aunt of Princess Cadance. Eventually, the Elements of Harmony restore Luna to her former self and she returns to rule over Equestria alongside her older sister. Princess Luna/Nightmare Moon is called by her respective early names Princess Selena/Night Mare in another depiction.
Princess Luna is introduced in the prologue of the first episode, depicted in a series of medieval-like drawings with a narration that says, “Two regal sisters who ruled together and created harmony for all the land,” and that “the eldest used her unicorn powers to raise the sun at dawn; the younger brought out the moon to begin the night.”
The narration continues by stating: their subjects, the ponies, played in the day but “shunned” the night and slept through it, which made the younger Alicorn 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.