Nightmare Moon was one of the main villains from My Little Pony: Friendship is Magic. She is currently set to be a member of the Villain League in the fanfiction series.
Nightmare Moon, also known as the Mare in the Moon, is an Alicorn pony first seen with blue armor and magic mist hair, and is the main antagonist of the first two episodes. A thousand years ago she was imprisoned in the moon by Princess Celestia, until finally her prison breaks and she is once again freed, whereupon she uses her strong magic to capture Celestia and prevent the sun from rising, to make Equestria a land of eternal night.
In an attempt to stop Twilight Sparkle and her friends from reaching the Elements of Harmony, the powerful artifacts that Celestia used to defeat her before, Nightmare Moon uses her magic powers to put a series of dangerous threats in their path to hurt, threaten, or lure them off their quest. At the end of "Friendship is Magic part 2" she is revealed as Princess Luna, Princess Celestia's little sister who is very remorseful of her own actions.
Role in the series
In the series, it is revealed that Pitch Black was technically responsible for the corruption of Luna during his reign in the Great Equestrian Fear War by creating the Nightmare Forces (MLP comics) as a backup plan if he should ever be defeated, promising to the forces that Luna would allow them to have a kingdom of their own. When he was defeated at the end of the war, the Nightmare Forces knew that that was their cue, and used Luna's jealousy and bitterness to tempt her into becoming Nightmare Moon. She even returns when the Villain League use corruption powder doused with some of Luna's dark side, allowing them to corrupt her into her dark state and fight alongside Discord and Queen Chrysalis. She will even make another return when she feels insulted by seeing that the Superiors' home planet's moon is inhabited, thus causing her to turn into Nightmare Moon, and declare war on the Superiors. Also, Nightmare Moon is later revealed to be a form of limited power which reduces her power to maintain control. But if she lets loose all limits, she becomes Earthlight Moon. And like Celestia's Daybreaker form and it's relationship with Solar Flare, if the two clash, they could potentially destroy themselves and cause disharmony.