Disclaimer: I do not own any of the characters or places in the story. I make no money off of this work of fiction and claim no rights to My Little Pony: Friendship is magic.
A/N: Just a short story I came up with after watching a Luna Tribute on Youtube. A lot of the backstory is pure fan speculation. Enjoy.
Princess Luna wandered the halls of the palace quietly, thoughts, memories flashing across her mind's eye. The worst part was what she remembered, which was... everything. She remembered the fight with Discord, the subtle magic he had used on her, the quiet voice he had used to convince her....
"Poor, pathetic Princess Luna. Do you really think that saving Equestria is a good thing? Don't you realize that all you're doing is protecting a land that doesn't care about you?" the Draconeques had drawled, watching the black Alicorn.
"Silence yourself, wretch! My sister and I are Equestria's rulers, and none shall harm--"
"Yes, yes, but honestly. Do you really think they love you? You poor soul. They sleep at night, Luna. They sleep and ignore the night time. While your sister rules the day, when everyone is awake, having fun. And have you noticed that there's no sort of moon festival in this pathetic place? I have."
Luna frowned, doubt showing in her eyes for the first time since she'd been dubbed the ruler of the Moon. In that one moment of doubt, Discord had struck. "Side with me, Luna. Together, we can destroy your sister and rule Equestria. But only together."
"I do not wish to destroy my sister..."
"Necessary sacrifice. If you don't do this one thing, then you'll be stuck in the night forever."
Luna nodded, unaware of the magic slowly working its way through her. A subtle bit of Discord's power, working its way into her to be noticed at a later date.
In the end, she hadn't destroyed her sister. But as they were turning Discord to stone with the newfound Elements of Harmony, the beast had just smiled and said, "Soon."
It wasn't long after that that the piece of Discord's corruption had made its self known to Luna. She had begun working in private to make herself more powerful than her sister. And finally, when the time was right, she'd struck.
The Summer Sun Celebration had always been a big day for the Princesses. They both had to look their best and be on time to whatever town the festival was being held. On this particular day, Luna used her magic to transform herself to look more like her sister. That in its self wasn't odd.
What was odd was the intimidating aura she gave off, to which passing guard ponies would shudder. Even as they flew to Ponyville, living creatures within a hundred yards would scatter off to the side.
When they had landed in the town and the celebration had started, Luna launched her coup.
"And now," Celestia had said, "I will raise the sun and we can rejoice in the longest day of the year."
Luna had waited patiently as her sister attempted to bring the sun up. Celestia had smiled at her and said, "Dear sister, I believe you have forgotten to lower your aspect. I cannot bring the sun to bear if the moon is in the way. If you would kindly lower it?"
"No." One word, one syllable, an even tone. It was enough to make ponies start murmuring.
"Excuse me?" Celestia was staring at her sister, unsure of what was happening, though she had a sneaking suspicion.
"I said no, sister. I will not be forgotten anymore. From this night on, I will rule! From this night on, there will be no day! From this night on, I am not your sister! I am Nightmare Moon! Bow before me, or be punished!"
"I think not. I know not who you are, or how you came to possess my sister, but I will not bow to you. Release my Luna or face my wrath."
"Do your worst!" Luna- Nightmare Moon- had stood, horn glowing, ready to attack her sister. Celestia, to her credit, didn't even bat an eye. Instead, she simply brought her own magic to bear, and with a simple spell summoned the most powerful weapons at her disposal.
The Elements of Harmony floated around the solar princess as she stared down the cruel parody of her sister's form. "Yield, Luna, and I won't have to use these."
"Fool! You think that those can defeat me? I helped you use them the last time! They are as bound to me as they are to you!"
"Are they really now? Laughter, Loyalty, and Generosity. Those were the ones you embodied. But no longer do you carry these traits. Let us test it then. You like the night and the moon so much? Then to the moon you shall go!" Celestia activated the Elements, watching as their stunningly bright colors had hit the being that used to be her sister. With a cry of anguish, Nightmare Moon found herself being flung through space, far quicker than she thought was possible.
As she hit the ground, Luna felt the being inside of her go dormant, though one final Nightmarish thought crossed her mind. Now we wait.