The outside was pretty calm as everypony just minded their own business, carrying along with their daily doing-nothings. (...I had to link this. I feel awful now)
This calm was shattered as the entire top of Twilight's tree exploded into a vast cloud of fire and smoke, the impending shockwave knocking everypony down. From this billowing cloud flung out some sort of black ball. It was Cosmos. Her face was stricken with panic while Celestia chased after her in the air with wings folded outwards like thin blades. Her eyes sparked with a thundering flame. The white alicorn concentrated some more magic along her wings and flung it at the opposing alicorn, who barely had any time to react and block the blast with her own wings. The strike proved too powerful, and Cosmos still found herself flung further away into the open plains between Ponyville and the Everfree forest.
"Princess, wait!" Twilight begged as she galopped towards the scene of combat.
"STOP this instant, Twilight Sparkle."
The unicorn did just that, but she suddenly shrunk to the size of a mouse, and her chest began to crush her lungs, straining her breathing. Luna had a very stern look that showed a mixture of anger and worry. The alicorn placed herself right in front of the lavender pony and stared her down, her own eyes mere centimeters away from the shying Twilight Sparkle.
"What is the meaning of this? You recreated Nightmare Moon?" she asked as she backed away, widening the gap."Did she come back to life?"
"I...I can't remember if she told me or not. She has been acting strangely ever since we found her in Ponyville."
Luna cocked an eyebrow.
"It's true! She didn't know how to fly nor how to use magic. She didn't even know who she was. She keeps calling herself 'Cosmos Lune' and keeps talking to herself."
The alicorn's mouth opened in astonishment, yet, despite her wanting to say something, nothing came out.
However, putting a hoof to her chin for deep thinking, she realized something very important.
"If what you say is true, then, perhaps...We must find them before my sister tries to eliminate Nightm-Erm...Cosmos Lune."
As Luna finished her sentence, another explosion occured on the outskirts of the city, although this one looked a like a spinning tower of flame. However, some sort of glowing blue wave rose up from behind some of the houses and smashed it down, extinguising it and bringing forth an immense cloud of smoke. The two raced to the scene while ponies all around began to seep their heads out of their hiding places. Spike was told to stay behind and tend to the house. He pouted and crossed his arms, but did as told.
"W-wait! GAH!" Cosmos screamed as she dodged some sort of bolt.
Said projectile zipped past the mare's head and melted a small patch of ground it touched. The entire plains were shaking at the anger and power being employed. Celestia's rage was seething, and her eyes were as white as the hottest of fires. The ailcorn started to fire her bolts off in quick succession, and Cosmos began to panic as she yelled with every dodge she made. The dark alicorn had to get into bizarre positions to avoid each and every projectile fired at her, her panic beginning to well up little by little.
"Kill that wench already! She aggressed us without reason! That's reason enough to het rid of her." the true Cosmos enforced.
"No! I can't kill. That's very bad. One isn't allowed to take the life of another without reason."
"She's trying to ill you! That's reason enough."
"No it isn't. I won't lower myself to such tactics. AH!"
The white alicorn had formed some type of spikey shield in front of her and used it to charge into the opposing foe, but missed. As a counter, Celestia found herself tossed away by a wave of ice. The blast was strong enough to send her darting towards the ground and slide along it, ruining the landscape and filling the princess' mouth with grass. Still dazed by this sudden strike of power, the alicorn lift her head to show a rather comedic show of sideways eyes and a mouth full of grass.
"Gentlemen." Comos thought to herself as she began to laugh loudly.
This comedy was short lived, as Celestia had already reganed herself and began to spin her head around while her horn began to glow brighter and brighter.
"Wh-what's she doing?"
"She's summoning a tornado. Stop her! Fire off a moon bolt or something."
"Ooookaaaay."
Cosmos abided, but she tried to aim for the ground underneath Celestia. As she fired off, her head involuntarily inched upwards, allowing the strike to smack the alicorn right in the chest. Her cry of pain screeched in Layda's ears.
"Oh no!" she teared"What did I do?!"
"You prevented her from performing that could kill you. Doesn't it feel good." the true Cosmos finished with a syllable of pleasure.
"NO! I don't like killing!"Cosmos clenched her head and began to shake it in refusal of having possibly killed someone.
This mental torture was short-lived as everything suddenly went white. Cosmos felt drowzy, groaning in pain as she tried to get back up. Her returning view allowed her to discover that she was now on her back with Celestia standing on her while being accompanied by several blades of magic aimed at her throat. The dark alicorn simply whimpered.
"I don't know how you returned, Nightmare, but you won't stay for long. I'll finish you, and this time, you won't have my sister to hide behind."
Celestia pulled the blades back. Her teeth gritting, her heart racing, and her sweat running, she was about to give a strike, but stopped suddenly. Her whitened eyes suddenly went back to normal, and her whole being was stricken with confusion. The dreaded and feared Nightmare Moon started to tear and sniffle. She had succumbed to the inevitable fear of death. She had actually started to cry. The only time that happened was when she was still more Luna than Nightmare.
"PRINCESS, WAIT!" Twilight shouted suddenly.
"Yes. Sister, do not execute Cosmos Lune!"
The two were galopping as fast as they could, the sudden panic causing Luna to forget the existence of her wings. Celestia just stared at both with as much confusion as before.
"Cosmos?" the white alicorn whispered under her breath.
"Yes. Princess...I...I'm out...out of breath." Twilight panted heavily.
She was almost panting like a dog. Luna, on the other hoof, kept her proud and elegant composture.
"I trust you have something to explain for THIS?!"
"AH!" Cosmos yelped, a sword getting a hair away from her throat.
"Wait! I can explain everything...or...atleast as much as I know about her." Twilight begged.
There was a long moment of silence where Celestia first stared at her student, then at the cowering Nightmare behind her. The princess looked up to the shining sky, then sighed out of irritation.
"Fine. I will do this, but she will be brought to the dungeons first. If anything indicates anything that could prove that this is all a little game, then she will be banished, and this time, it will be forever!" she glared at Cosmos, who immediately shrunk a little lower into the ground."Now get up!"
The alicorn scampered up to her hooves and lowered her head as she continued to sniffle and tear. Luna and Twilight both looked at her with pity, but a sun-ray shone over her face and created some form of mirage that could only be qualified as terrifying. Creating some form of over-lapping mirror effect, another face appeared. It looked like the original Nightmare's face smiling evily at this outcome. It was a brief glimpse, but this made both Twilight and Luna look at eachother in both fear, confusion, and curiousity. The trip to the castle would be a quick one. Teleporting wasn't too hard, especially for Celestia preferring to teleport everypony to Canterlot herself. This would be a good way to keep watch on "Cosmos Lune".
"Heheh. It is nearing completion." the true Cosmos thought.