This fanfiction is based off of this scene from the My Little Pony episode “Do Princesses Dream of Magic Sheep?”:
“I created the Tantabus to give myself the same nightmare every night,” panted Luna, “to punish myself for the evil I caused as Nightmare Moon!”
“But why would you do that?!” cried Fluttershy.
“To make sure I never forgave myself for how much Equestria suffered because of me!” sobbed Luna. “But it seems I have not learned my lesson, for now I have only made you suffer more!”
“But that means you might just be the key to stopping all this!” said Twilight
“Yeah!” put in Rainbow Dash. “If it gets strong because you feel bad about what you did as Nightmare Moon, then you just gotta stop feeling bad for what you did!”
“How can I forgive myself? I am no better now than I was then. My creation is about to turn the world into a living nightmare!”
“But look at what you’re doing!” said Twilight. “Nightmare Moon would’ve wanted the Tantabus to turn Equestria into a nightmare! You’re doing everything you can to stop it! Don’t you see? That proves you’re not the same pony you were then! Everypony who knows you knows that Nightmare Moon is in the past! We all trust you, Luna! Do you trust us enough to believe we’re right?”
“I… can’t!” screamed Luna.
With a force like an explosion, Princess Luna vanished in a flash of white light.
But she wasn’t the only one gone.
The Tantabus, seconds away from crossing into the real world, had vanished too.
In the silence that followed, Fluttershy asked Twilight the question every pony was thinking: “Where’s Luna?”
“I don’t know,” said Twilight, unable to grasp what had just happened.
The rest of the night passed in dreamless sleep.
In the morning, Princess Celestia woke to find Luna’s bed empty.
Other than the envelope on the pillow, Luna’s room was as it had always been.
A single word — Sister — was written on it.
Opening the envelope, Princess Celestia read the letter inside:
I am sorry.
Nothing I say will convince you that my powers are a curse. Not a gift.
I know you feel I have a duty to help other ponies.
For now, I am the pony I need to help.
I have decided to get away.
I do not know when I will be back.
Give my best to Twilight and her friends.
Always your little sister,
The art of Luna’s bedroom comes from here.
Luna’s letter to her sister Celestia is inspired by George’s letter to his brother Billy in the 2010 film Hereafter.