Disclaimer: I do not own My Little Pony.
Celestia P.O.V
The moonlight seemed harsher tonight. Luna had not returned in a day, but the moon was smoothly brought up by somepony in the distance. I knew my sister was okay, because only she and I could raise moon or sun so easily. She probably only needed time to think and would return in the morning, and everything would fall back into place.
At least, I hope it will all be fine, yet I couldn't help seeing an omen in the way the moon's silvery glow bled into the lake near the castle. With that happy thought, I retired for the evening and went to bed.
Come dawn, try as I may, the moon would not go down. My sister had not lowered it, and some force was preventing me from doing so.
I turned and saw a dark shape approaching. Luna was back and my anxiety dissolved. A few seconds later, I realised my problems had only just begun; the pony wasn't my sister.
With a look of contempt, the fearsome looking mare landed on the balcony.
"Who are you? How dare you intrude in the castle!"
She glared at me with eyes that, even though the pupils were smaller, were exactly the same colour as Luna's. The way this newcomer stood…
"Sis?"
Luna P.O.V
My resolve almost broke. There was so much disappointment and angst in her voice. But I remembered that she had taken so long to recognize me, her own sister.
"What is this madness Luna?" her tone made me even angrier. It was as if she was mother, scolding me.
I cocked my head and let loose a maniacal laugh. I was no longer her little filly, and would never be that again. Without further delay, I charged.
Her horn clashed with mine, yet she was reluctant. I pressed on as the air surrounding us was charged with ozone, our powers rendering each other ineffective.
Hearing the commotion, our guards, sorry, Celestia's palace guards, rushed in. I instantly cast a force field of dark energy to prevent them from disrupting our battle.
"Did you just use dark magic?" hissed my sister.
I ignored her and used her distraction to whack her across the floor. She neighed in surprise and attempted to counterattack. Then, I used my favourite spell of the ones Sombra taught me. Celestia's horn became speckled with dark stones. Her brow furrowed in concentration, she tried to cast another spell. It was futile; my magic had rendered her horn useless. I prepared to attack once more, I wouldn't kill, but I would take her wings, destroy her horn, lock her up in the deepest dungeon, wait…
I stared at my sister; I loved no one like I loved her. Her eyes weren't pleading, they just asked a simple question: What will you do?
"Oh! Tia, I'm sorry, it got out of hand I can never hurt you!"
"But you will my dear", came the voice I didn't want to hear. "You will" It was Discord materializing in front of us.
"Never. I've changed my mind"
"But, you see, we had a deal. No one gets out of a deal with me." I knew what he was going to do a second before he did it. Coloured rings dilated in his eyes. I was unable to resist, my power spent on the duel. It was wrong; I would not give in, never. His hypnosis…
Celestia P.O.V
"You can do it dear sister, he won't control you!" I shouted, trying to spare my sibling. It was already too late. Luna turned and spoke words that would forever haunt my nightmares.
"I am Nightmare Moon. You are no sister of mine."
That one was the longest one yet! Hope you liked it!
As always, constructive criticism is welcome.