The Unforgotten Tale of Nightmare Moon

There can only be one..

The night princess, her thoughts now possessed by Nightmare Moon, walked back into the castle. Throughout her argument with Nightmare Moon, her older sister, Princess Celestia, started to grow very worried. She searched for her sister in the big castle, but had no luck in finding her. "Luna!" She called, a worried tone in her voice. She was starting to panic.
"Yes sister?" Luna replied, standing in the entrance of the castle.
"Don't ever run off like that again!" Said Celestia, rushing over to her and putting her fore arm around her protectively. Luna glared up at her sister, "Why would you care?" She asked, stepping away from her hug and walking past her. Nightmare Moon was already starting to get to her..
"Don't you take that tone of voice at me, young lady!" Growled Celestia, annoyed by the way Luna is treating her. 'She doesn't normally act like this...' The sun princess thought, slightly worried.
"Why? Are you too precious to be talked to that way?" Luna growled back before storming off into the throne room. When she entered the room, she jumped behind her throne, growling and twitching with rage. 'Luna..it's time to get nasty!' Nightmare Moon shouted inside her mind. Princess Luna smirked and waited for her sister to enter the room.
"Luna?! Come out now and apologize!" Called Celestia, walking into the throne room and slowly towards her sister's throne.
"Not another step!" She growled, coming out from the back of the throne.
Celestia looked at her sister weirdly, "Luna! I-"
"Did you really expect me to sit idly by while they all bathed in your.. precious light?!" She mocked, cutting her off. Princess Luna walked onto the bridge that was between the two thrones. "There can only be ONE princess in Equestria! And that princess..." She shouted, glaring down at her sister, "..will be ME!"

Celestia's POV:
I knew this day would come.. I knew right from the start Luna started growing into a young adult. She started to become sensitive and unhappy. I once went to visit her after a day out, because she looked very upset when everyone crowded around me. We were used to a big crowd, so I didn't really understand why she looked sad. I thought she loved talking to fellow ponies. That's when I realized she was alone, as everypony would always come to me and not her. I knocked on her door and opened it with my magic. Her top half of her body was leaning on the inside window ledge, her face against the cold, painted stone. One of her forehooves was covering her face as tears streamed down her cheeks. I looked at her guiltily before saying, "Oh sister I am so sorry.."
"Celestia, get out!" She shouted at me, turning her head in my direction with an angry expression.
I gasped and returned the anger, "Luna! Don't speak to me like that!"
"I said GET OUT!" She screamed in rage, standing up with her wings spread wide with angry tears of hate streaming down her face.
I shouted back, "Luna!" but that was a mistake. She slammed her hooves down and screamed at me again. She looked so scary that I quickly shut the door, tears in my eyes, and walked away from her room.

Princess Luna eyes glowed white as she raised her front hooves and stomped on the front of the bridge, crushing it. Then all of a sudden, a dark shadow overtook the room and a crack went up the wall behind her and into the window..
The crack reached the stained-glass window and broke it, shards of glass crashing and flying to the floor. Celestia's bright sun shone through the broken wall. But it was only there for a second..
Princess Luna lighted up her horn with her dark blue aura and flew into the air, raising her moon as she went higher. She kept raising the moon until it covered the sun and turned into an eclipse. The eclipse sent out dark beams of light through the broken wall.
Suddenly shadows surrounded her. Luna's horn was still glowing and she placed her two front hooves on her head. Her eyes grew big. The shadows sealed tightly into a ball, her inside...
The ball grew bigger and brighter around her until the room flashed white and the ball broke.
The ball revealed a tall, dark mare with big, flowing midnight-blue hair. She wore a helmet and her horn was longer than Luna's. She opened her eyes and they looked demonish, green and the iris was thin and small. She started laughing maniacally, her teeth long and sharp. She landed on the broken bridge with her powerful, armored hooves. She was covered in dark blue armor, a moon on the front. She continued to laugh evilly, closing her eyes and throwing her head back. It was none other than..

Nightmare Moon..