Kaylin in Luna's form.
Kaylin was a 14 year old girl who loved ponies, One day she found a strange portal that brought her into Equestria. She found herself in the body of Princess Luna. She desperately told Princess Celestia that she wasn't really Luna. Celestia found the portal and shut it off, Celestia believes that Luna lived the normal human lifespan and died after about eighty years. She still today rules as Princess Luna.
Nightmare Moon[]
Shortly after arriving in Equestria, Kaylin began to go under changes, near confessions of who she really was, terrible fits that nearly brought down the castle, and finally, a rage that even Celestia couldn't calm. Kaylin changed into the monster known as Nightmare Moon. Nightmare Moon was created from the alicorn power of Luna and the evil of human sins, she was so powerful that Celestia had no choice but to banish her for a thousand years to the moon.
Revenge[]
She fought the pretese of good and evil for a thousand years, becoming more and less human each year. Finally, evil won the duel and she waited, the day of the Summer Sun Celebration when Celestia was busy, her defenses weakened and she struck, Celestia as good as gave up, she didn't resist when she attacked, but instead of finishing her off as she lay, weak and defeated at her feet, a part of Kaylin surfaced, remembering how kind Celestia always was to her, Nightmare couldn't shake the moment of guilt and teleported her to the sun instead of killing her, thinking it as proper punishment for banishing her, forgetting to put the protective enchantment around the surface in her confusion.
Defeat[]
She continued with her plan, not bothering to check if the sudden change of events would alter anything in the path she was taking and took Celestia's place at the celebration, the ponies reacted as you would expect, except for the quirky and to her definition " nosy " student of Celestia's whom with a group of five other ponies, found the location of the elements and set out to get them. Nightmare used several technequies to try to stop them but the six ponies prevailed, at the ruin that they were located, she toyed with Celestia's student and destroyed the elements. However, she had messed with the wrong ponies and they resurrected the elements and tore away the guilt of human sins and reformed Kaylin with a new and pure innocence. Celestia managed to escape the sun due to the lack of the barrier and forgave Kaylin of her sins.
Luna Eclipsed[]
Kaylin felt that she was getting neclected of her title as princess due to the fear of ponies whom still believed that she was evil still. So Celestia allowed her to travel to Ponyville on her night off, unfortunately Nightmare Night. She quickly became frustrated with the ponies fear of her in Canterlot and went to Ponyville to try her luck elsewhere, sadly, Ponyville was no better, she got frustrated again and had a tantrum about it, she tried to leave but was stopped by the persistent student of celestia, Twilight Sparkle. She helped Kaylin gain the friendship of Ponyville's residents and later, all of Equestria.
A Canterlot Wedding[]
After lowering the moon the morning Twilight and her friends arrived, she returned to the castle, unfortunately, Queen Chrysalis awaited her, Kaylin tried to sound te warning but Chrysalis knocked her out and transported her to the caves beneath Canterlot. After the whole fiasco with the changelings, she woke up and wandered out, she found the wedding procession outside and flew down, pretending that she didn't know anything that happened. Later, Celestia told her the story and consumed with guilt, she locked herself in her room for four days, Celestia finally convinced her that there was nothing she could do and they continued their rule together.
Aftermath[]
Today Kaylin still rules with Celestia and will for a very long time, she agreed with Celestia to make Twilight Sparkle an alicorn and she raises the moon and stars every night, look out your window, you might just see her beautiful night.