Okay, look, before you post some rude comment about how bad this is:
I only wrote this because I'm desperate to get out of writer's block. Plus, I really wanted to explore this concept.
So if you have nothing nice to say, then please be quiet. Thank you.
Luna took a break from her restless pacing and sighed, flopping down onto her side. Shortly after being banished to the moon, she had become her normal self again. But she knew the moon wouldn't be able to hold her forever-and she wouldn't be able to control herself if Nightmare Moon took her over again. There had to be something she could do-something that would save all of Equestria; maybe even herself. But what could she do? Until the right time came, she was completely blocked off from the world. She couldn't see anypony, or interact with them in any way.
She started pacing again, wondering if there was any hope. She knew her sister wouldn't be connected to the Elements of Harmony forever, so what would stop Nightmare Moon?
Luna stopped in her tracks. She had an idea! She didn't think it would work, but it was worth a try. Her horn glowed faintly, only having a tiny bit of power during her exile.
Please, give me this chance! she prayed.
Suddenly, a beam of light exploded from her horn and enveloped her, temporarily blinding her. The light split in half, and one half went inside of her. When all of this was over, Luna noticed a trail of white light silently gliding towards Equestria. She also felt as if something inside of her was missing. She knew it was working-but now it was up to the ponies down there to determine the outcome.
Celestia smiled down at the little filly that was now her student. The filly was bouncing up and down excitedly, overjoyed at the events.
"What is her name?" she asked the parents.
"Twilight Sparkle," her father replied.
"Twilight is a very talented unicorn," Celestia replied. "I haven't seen magic like this at such a young age for years. Not since..." Celestia shook her head, trying to clear those memories. "Twilight will be well taken care of."
"Yes, of course, Your Majesty," Twilight's father said a bit hastily. "I'd never doubt you, Princess."
Celestia wasn't paying attention though. She was focused on the happy filly Twilight. Something seemed strangely familiar about her, but she couldn't put her hoof on it.
While Twilight was asleep one night, Celestia decided to go to the castle library and do some research. As she went through many scrolls about spells that only alicorns could perform, she came across something that caught her eye.
The Soul Splitting Spell
This spell cuts a pony's soul in half and puts one half in the body of a new pony.
That was all she needed to see. She now realized that her suspicions were correct-Twilight was half of her sister's soul. She smiled to herself, knowing that her student would be the one powerful enough to defeat Nightmare Moon and bring the original Luna back again. But for now, a part of her little sister was with her-and she swore to remain faithful to her always. She'd never make the same mistake again.
As she passed by the room where Twilight was sleeping, she paused and gazed at the purple filly.
"I love you, Luna..." she whispered.