Nightmare Moon is one of the main antagonists in My Little Pony: Friendship is Magic. She appears as a villain in the two-part pilot episode "Friendship is Magic" also known as "Mare in the Moon". Before that, she was known as Princess Luna, and is the younger sister of Princess Celestia. It is unclear if "Nightmare Moon" is an Entity which possessed Luna or not; the show seems to lean that Nightmare Moon is Luna's alternate personality but the comics state that Luna was corrupted during her time on the moon.