It is said that Nightmare Moon appeared from the anger and sadness in the heart of Luna . She had come with the intention of gaining everlasting night which had gained, but with the power of the elements of harmony, Celestia imprisioned her in the moon for 1000 years. After 1000 years, nightmare came back to reclaim her everlasting on the day of the 1000th summer sun celebration, she took princess Celestia a hostage and claimed her everlasting nightto be stopped by Twilight sparkle and
her 5 friends, Applejack, Rarity, Rainbow dash, Fluttershy and Pinkie pie. She was again, banished, but this time she was sent to an alternate dimension. On the dimension, Nightmare slowly regained her form and strength but not her powers fully. Now she is back and is trying to once again get everlasting night and powers back.
Nightmare has one of the most funny PJ reactions ever, at least in the roleplayer's eye. She has a normal mane with rainbow color, a earthpony status and becomes Shymare moon, in this case, she becomes shy and nice
Nightmare was the first ever to create the inside gag on the network which is 'This is x... And you just fell in it' With the post 'This is poison joke... And you just fell in it'.
Nightmare is in love with Charlie and she helped him get rid of Red and Blue
Nightmare's horn tastes like ground up moon rocks, and as it is known, those are pure posion.