Once upon a time, about 1000 moons ago, lived two sisters who ruled over the land of Equestria: a land full of cute, playful ponies. The two sisters were both mares with unicorn horns and pegasus wings, and their beauty shone through the rays of the sun and the light of the moon. Aside from that though, the ponies were as different as night and day itself.
The eldest of the two, Princess Celestia, was a tall, dove white mare. Her mane and tail were long and flowed in the gentle breeze, shedding glitter as her baby pink hair blew casually. Her peached iris gaze was gentle yet firm, like that of a mother, for In a way, she was like a mother. She had no castle, only her special spot in the forest where the sun could shine upon her, and she could sit on a throne made of flowers crafted by the loving equines who decided she would rule to begin with due to her maturity and grace.
Beside her throne was one made of gorgeous lilies and soft vines, and this royal seat belonged to her little sister, Princess Luna. Standing shorter than her sister, who was large like a mighty horse, she came to be just taller than the average pony. Her shorter mane and tail were a pale indigo, and her coat that of the oncoming night sky itself. Little stars could be seen in her cutely bobbed mane if you were awake during the night, which is when she would take her turn to rule. The beautiful space rock loomed over the shorter princess as she pranced around alone in the fields and forests, looking after her sleeping friends to make sure they rested well.
As time went on, Luna began to get increasingly aware of how the ponies praised her sister and the sun, and took little appreciation for her and the moon, even those previously cursed with sleepless nights. This realization birthed a jealousy in Luna, especially as after she confided in her sister, Celestia merely told her that the ponies loved her too. In the pale mare's heart, she truly believed her ponies loved her sister too, but Luna could not see things as she did, for it was not true.
Feeling like she was in an unbeatable, constant shadow with nopony to turn to, her jealousy grew until it consumed her, causing her to completely isolate herself from her loving sister and her duties. One day, when they were supposed to switch places and Celestia was meant to raise the sun as Luna lowered the moon, she refused, causing an eclipse to shroud the land, and the littler sister, in suffocating darkness. This darkness bathed Luna in her own fears, and making her sister finally confront the waking nightmare that was now her sister; Knightmare Moon, Empress of The Night.
This change frightened her sister as before her eyes, her little sister matched the sun loving princess in build. Her coat was now the slick oily black of a crows, and her teeth became pointed and her tongue sharp. Her twilight gaze of slit pupils had a tunnel vision on her sister, causing all her hatred and pain from neglect to rain down on her. Stars still shimmered in her now flowing hair, looking like the night sky itself, and a silver helmet was mounted on the terrifying pony's head, making her head truly resemble the dark side of the moon itself, with a clearer sister moon that her horn mimicked. More silver and onyx armor adorned itself as her sister prepared to take her rightful place as ruler of all of Equestria, and the one their subjects would finally adore.
It pained Celestia to see her sister like this, crushing her heart, but she could not back down. With an eclipse no crops could grow, the land would turn cold, and the ponies could not see so they could not play. Still pleading for her to lower the moon only to be met with cackles of insomnia, Celestia did the only thing she could. Recalling a spell their shared mentor, Starswirl the Bearded, had taught them years ago, she remembered everything she loved about her sister. She loved Luna's kindness to those in need, her loyalty to her duty to the moon, her honesty that she always generously shared with her, the way she would laugh in the face of danger whenever their land was threatened, and that magical, loving feeling any sister felt towards their sibling. Painfully gathering these emotions and memories, she used them in a spell to banish her sister to the moon for the next 1000 years.
This action haunted Celestia anytime she was now tasked with raising the moon and sun alone, and everytime she saw the mark of that mare in the moon, even the lavender streaks that now stained her purely pink hair. She didn't want to do it, but at the time, Luna was manic and without reason, and there was nothing else she could do. Now, she sits in the forest alone and solemn next to her little sister's wilting throne, waiting for her return as her little subjects barely care for her sister's absence. It seemed, Luna was right, and she regrets her ignorance every day.
Fortunately, in the wake of Luna's return, Celestia's own pupil, a unicorn named Twilight Sparkle, will be the one to help defeat the poor nightmare.