We all know the Tale of the Two Sisters.
How 'Once upon a time in the magical land of Equestria, there were two regal sisters who ruled together.
To do this the eldest used her Unicorn powers to raise the sun at dawn.
And the younger brought out the moon to begin the night.'
And about how Princess Luna became jealous of everyone loving and relishing in her sisters Day while they slept through her beautiful Night.
So one night she refused to lower the moon to make way for the dawn.
Soon making that jealousy turn her into a wicked mare of darkness Nightmare Moon who had to be banished to the moon by Celestia of 1000 years.
Well... What if there was another reason why she didn't lower the moon in time for the sunrise?
And what if Celestia was only so mad at Luna for not doing it when she should - meaning the hatred and darkness built up inside the younger sister, because it had been happening for a while?
And what if that reason was because Luna was being distracted by something else?
Or rather... someone else?
Princess Luna was singing a lullaby to a little filly who was as pale as the moon with a crescent moon mark on her forehead just under her horn.
"Moonlight... starlight... darkness falls
From the shadows owls call
All this shall be yours someday
Till then sleep
Dream the night away..."
"Goodnight my little Selene," Luna said gently nibbling on the filly Alicorn's wings to show she loved her and kissed her forehead.
Just then there was a banging of a door being thrown open and Celestia's voice calling out for her.
"LUNA!"
Not wanting her daughter to wake up, or her older sister to know about Selene (she didn't deserve to know she thought) Luna placed an Everlasting Sleeping spell, meaning time stood still for the pony it was cast over as they slept, over the filly.
Then after another kiss, Luna went to see what was wrong, hoping this would be a quick conversation.
As she stepped out into the hall, she could see Celestia pacing around, every so often looking to the moon.
"Luna," she said fiercely in a form of greeting "Why have you not lowered the moon yet? This is the 5th time in two weeks you haven't lowered it when it was meant to go down."
"I was busy with something else and lost track of time sister," Luna said going over to the window to do her job, but Celestia caught her with what she said next.
"Lost track of time? You seem to be doing that a lot lately. What you think ponies will notice the moon more if its out longer and longer every night?" she asked sounding like the brat Luna secretly believed she was "What's got you so busy anyway?"
"That is none of your business sister," Luna said pointing a hoof at Celestia.
"It is my business if my ponies suffer because of it because the crops aren't growing because the sun's not getting to them to help them grow."
"Your ponies!" Luna asked yelling a little. "We are ruling this land together! Those ponies are just as much mine as they are yours!"
"You know what forget it, I should have done it myself," Celestia said moving to the window to see what she was doing.
"Not another step." Luna said coldly "Did you think I'd sit idly by while they all basked in your precious light?"
Slowly she went to the top of the stairs keeping her eye on her sister along the way saying "There can only be one princess in Equestria. and that princess WILL BE ME!"
Screaming this Luna brought her hooves down onto the railing smashing it to pieces.
Then all the hatred and jealousy and stress for caring for Selene that had built up over these last few months came out of the bottle, surrounded her, and turned Luna into a mare of darkness calling herself Nightmare Moon.
A battle and a banishment to the moon later, Celestia placed the stone Elements of Harmony back onto the stand and left.
Nopony knowing of the little filly that was soundly sleeping in a room just down the hall, who would stay asleep until her mummy came to wake her up.