It was night in Equestria. Luna was in down, deep down in one of the castle chambers. It was never used, so it was perfect.
Everything was ready. The circle* was drawn, the potion was mixed. The spell book was open, held in the air by Princess Luna's levitation spell.
Celestia had warned Luna about the black magic book. She had said it was only for research, not for practice.
Luna stood on the edge of the circle, and began the incantation**.
Signum a porro
Ego permissum vos iam vestri vox fluant
The circle began to glow white on the marked lines.
Ex infusco vorago quod everto incendia
Vestri vox est quis Ego votum
The liquid in the vial changed from a light blue to black.
Luna took the potion, pressing the vial to her lips as she swigged down the fizzy and bitter liquid.
Orior oriri ortus iterum , vestri vicis has adveho
Permissum is vomica iam exsisto laxo
Obscurum ex a vicis pro
nox noctis equus vadum exsisto existo iterum
Luna watched the circle as a shadowy form arose from it. It was not a pony, nor any creature that she had seen in Equestria. She backed away, bumping the table that she had used to make the potion. A glass bottle on the edge toppled to the floor with a crash. The shadow came forward, and Luna stood, frozen in place now, horrified.
Nightmare Moon cackled, long and loud. It felt good to be free. Though, this form...It was unusual, but it would do. She sifted through the thoughts and memories of the body she now had, taking in the history and things she immediately needed to know.
She made her way up the stairs and out of the chamber. She passed through the throne room, taking in the place where she would soon rule.
* * *
Princess Celestia awoke just before dawn, as she always did, so that she could raise the sun and prepare for a new day. She quietly left her chambers, making her way up to the tallest tower in Canterlot. It was where she and Luna almost always went to carry out their respective duties.
Luna was already there, surprisingly. Celestia sensed that something was amiss, but she strode over to her sister. She had been staring out of the one window in the spacious tower. As Celestia came up to it, she saw that the moon was higher in the sky than it should have been at this hour.
Suddenly, Nightmare Moon whipped her head around, catching Celestia off guard and slicing into her shoulder with her horn. Celestia stumbled back, her eyes wide.
Her shoulder was stained crimson as the blood began to spread. Celestia backed up, threatened by her own flesh and blood. Nightmare Moon watched her limp backwards until she reached the wall on the other side of the tower.
Nightmare Moon put her head down, her horn forward. She spread her wings wide, and her front hoof scraped repeatedly on the ground. Her eyes pierced into Celestia, menacing.
Celestia saw Luna preparing to attack. “Don't do this.” she pleaded with her sister, as she lowered her horn as well. It began to glow as Celestia prepared to defend herself against her sister.
“THE NIGHT. WILL LAST. FOREVER!” she yelled.
Nightmare Moon charged, clearing the distance between them easily. Celestia focused all her power, everything she had, into doing something, anything. She couldn't think of what to do. Her little sister, little Luna, had turned upon her.
The Elements of Harmony, Celestia realized. If anything could save them both, it was the ancient lore. Celestia fought to remember the spell, the one that would call upon the Elements of Harmony—it would take a great deal of strength and concentration...but she had to do it. For Luna.
Luna was upon her, and Celestia braced herself for impact...but it did not come. There was a bright flash as horn met horn, and when Celestia looked up...Luna was gone.
“Luna?” she called out. “Luna?!” she glanced around in vain, when she saw the window again. Something made her look, made her check...
Celestia looked at the moon, still high in the sky...and saw a shadow of Luna indented upon it.
“What have I done?” she said, sliding to the floor, her eyes still fixated on the moon. “What have I done?”
*This is Latin and roughly translates to:
Sealed away from long ago
I let you now your powers flow
From blackest pit and demons fire
Your power is what I desire
Rise again, your time has come
Let this curse now be undone
Darkness from a time before
The Nightmare shall arise once more