Nightmare Moon

Nightmare Moon is what Princess Luna became when she tried to make the night last forever. She was imprisoned in the moon for 1000 years, and is known as 'The Mare in the Moon.'

Nightmare Moon, Queen Chrysalis, and other major villains in My Little Pony: FiM are based off of the Seven Deadly Sins.

Princess Luna's transformation into Nightmare Moon was caused by her anger about Princess Celestia's decision to reduce the night's length to the timespan where really everypony was sleeping.

Nightmare Moon is actually her normal form. Princess Luna is her "powered down" form due to the use of the Elements of Harmony.

Princess Celestia didn't banish Nightmare Moon the day she attacked. It was nighttime for more than a day until Princess Celestia brought herself to banish her sister.

The reason why Princess Celestia didn't use the Elements of Harmony when Princess Luna became Nightmare Moon is that she didn't have five friends to help her.