Nightmare Moon also known as Night Mare Moon is the main antagonist of the pilot episode of My Little Pony: Friendship is Magic.
1000 years ago there were two regal sisters who ruled Equestria. The eldest (Princess Celestia) raised and lowered the sun, and the younger (Princess Luna) raised and lowerd the moon, thus the two sisters were balancing The Day-Night Cycle and harmony in Equestria. the ponies played through-out the day the elder sister brought, while the ponies slept peacefully through the younger sisters night in wich the younger sister did not have anyone to socially interact with. One day the younger sister refused to lower the moon to make way for the dawn but the bitterness in the young one's heart had transformed her into a wicked mare of darkness Nightmare Moon, She threatened to shrow the land in eternal night. But not before the Elder sister got hold of the Elements of Harmony and banished her permenantlly in the moon, and took the job of controlling the moon along with the sun.
It was not until the Summer Sun Celebration when Nightmare Moon had returned to seek vengence. Twilight Sparkle and her new friends went into the Everfree Forest to a castle to retrive to elements to stop Nightmare Moon, as the retrive the elements she turns into a blue mist (the same mist as her mane and tail) and makes illusions to try to stop them. when they get to the castle Twilight Sparkle tries to stop Nightmare Moon on her own and she looses and thinks that all is lost suddenly she hears her friends and they use the elements to turn Nightmare moon back into Princess Luna.