At that moment, the darkness in Luna's heart manifested and consumed her, transforming her into Nightmare Moon, a black mare of pure darkness. Not only did she refuse to lower the moon, but also challenged Celestia for the sole rulership of Equestria. One day, Luna finally became fed up with Celestia's success. However, as time went on, Luna observed how the ponies would be awake to enjoy the day and love, admire, and appreciate Celestia's work, but would always 'shun' the night by sleeping through it, causing her to become resentful of her subjects and jealous of her older sister. Together, they kept the balance perfectly.

Every morning, Celestia would raise the sun, and every evening, Luna would raise the moon. Long ago, Luna and Celestia were the diarchs of Equestria, a land inhabited by ponies. In the comics, it was a pre-existing being that was born on the Moon but in the series it just Princess Luna's alter-ego.

The origins of the Nightmare itself vary depending on the source. 6.1 My Little Pony: Friendship is Magic.5.2.2 My Little Pony: FIENDship is Magic.5.1 My Little Pony: Friendship is Magic.Like her original form, she was voiced by Tabitha St. As Nightmare Moon, she acted as Celestia's archenemy until she was restored to her normal form by the Mane Six. She was the corrupted form of Princess Luna, the younger sister of Princess Celestia who was consumed by her own jealousy toward her older sister. Nightmare Moon is a major antagonist in My Little Pony: Friendship is Magic.

~ Luna right before transforming into Nightmare Moon. Where are we? Why did you and Celestia disappear?) Did you really expect me to sit idly by while they all basked in your precious light? (Twilight Sparkle: Precious light?) There can only be one princess in Equestria! And that princess… will be ME! NOT… another step! (Twilight Sparkle: Princess Luna! I don't understand. You little foal! Thinking you could defeat me!? Now you will never see your princess or your sun. From this moment forth, the night… will last… forever! (laughs) Remember this day, little ponies, for it was your last.

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