Night had descended upon the human world, but the sky was bright enough for Twilight Sparkle. Through the spiraling tower's sole octagon window, she could see the night sky as it was brightened by tracer rounds, explosions, and the occasional racing aircraft. The only thing obscuring her view was the dark black throne mounted directly before the window.
"You are a weak, incomplete mageling, Sparkle. To believe Celestia thought you fit to be a princess. That worm Sunset is more fit then you for such office."
The throne's occupant spoke, in cold, low voice that never the less dripped with malice
"But it need not be that way, Sparkle. Within you rests unmatched power, stifled by a weak teacher, one who prances through shadows, one who deceives you at every turn, twisting you into a hollow, wooden puppet."
"I can complete your training, Sparkle. Make you mage you were meant to be. There will be no lies between us, my student. Only the cold, painful truth."
"What good has Celestia's training ever done you, Sparkle? Gaze out the window to see where her ways lead."
"You see Twilight, your pitiful United Nations band has already failed. It was I who allowed the location of this crystal meth processing plant to fall into their hands. Everything that has occurred, has occurred as I have ordained it."
The throne swiveled around, revealing the occupant to be the sinister Generalissimo Sombra. He was the very picture of menace, wearing a dark suit decked out in self awarded medals, a black cape wrapping him like a cloak, and dark circles around his eyes. "You have failed, Sparkle. You have only one option." He then produced Twilight's lightsaber from his sleeve. "I am defenseless. Take your weapon and strike me down with all of your hatred. Then your journey to the dark side will be complete."
The elevator doors opened with a ding behind them. "Ah, Sunset, so good of you to join us." The wayward student of Celestia was clad in a jet black suit of riot armor with a similarly black cape going down the back. "You are just in time to watch the alliance die." Sombra croaked while pressing a button on his throne. "Fire when ready commander."
Miles away from the throne room, aboard a battleship cruising the ocean, Captain Snails, clad in a blue and gray camouflage jumpsuit and wearing a black beret, signals the technicians to begin the launch process. A few minutes later, missiles arched up from the ship, streaking across the sky to the battlefield.
Twilight could only watch in horror as the missile blast annihilated large swathes of the United Nation forces. "NO!' She screamed, calling her blade back to her grip with the power of magic, igniting it and swinging to decapitate Sombra. She found her purple blade blocked by Sunset's red blade as Sombra cackled with sinister delight.
Outside, Rainbow Dash pulled herself out of the dust, her ears ringing. She took a moment to recover her laser pistol, then surveyed the area. "AJ! Where are you!?"
"Over here Dash!" Yelled Applejack as she climbed out of a ruined tank. "Ah reckon we'd better retreat. We just don't have enough forces to keep fighting after that missile strike."
"But what about Twi!?" Rainbow shot back. Before Applejack could answer, an Imperial tank rolled up, and out popped old reliable Spike! He climbed out, one hand splintered apart into the wide variety of tools he found his robot body in possession of in this new world. He was followed by the sleek silver body of Rarity. "Mistress Dash! Mistress Apple! Hurry now, Spikey Wikey and I have commandeered this vehicle to conduct a hasty retreat!"
Once all four were inside, Rainbow pushed Spike out of the driver seat. "Buck that! We can use this baby to slip past the rest of the Imperial troops and get Twilight out of that castle!"
"Oh my! Out of all the dangerous things you've done Mistress Dash, this is the most Suicidal! Possible! Thing!"
Back in the throne room, Twilight had gone on the defensive against Sunset, and hidden in the throne room's many dark shadows. "I will not fight you Sunset."
"Your emotions betray you Sparkle. I can feel the anger in you." She replied, searching with both her eyes and her magic.
"As do yours. There is still good in you Sunset. Come back to Equestria Sunset, Celestia will forgive you."
"You are wrong Sparkle, and you know it. You're hiding something from me, I can feel it squirming in your mind." Sunset stopped. "Ah... Brother. You have a brother. If you will not submit to the dark side, perhaps he will."
That proved to be enough. Twilight burst from the shadows, battering Sunset with heavy blows from her lightsaber. Shimmer's defense crumbled, and soon she lay broken on the floor, burn wounds criss crossing her, heavy breathing escaping her lips. Twilight stood to strike the killing blow, when a haunting cackle reached her ears.
"Good! Good!" Sombra applauded, crossing the floor of the throne room in powerful strides. "Now, strike her down and take her place at my side. Then your journey to the dark side will be complete."
Twilight looked to the wheezing, broken form of Sunset Shimmer, then felt resolve wash over her. "No. You've failed your highness. I am Celestia's Faithful Student, like Sunset Shimmer before me."
Sombra's face twisted and coiled into a cold look of raw hatred. "So be it... student."
Burning bolts of electricity arced from Sombra's hands, striking Twilight and driving her to the floor, where she writhed in agony. "Sunset, help me!" She cried out.
Shimmer had pulled herself off the floor, burned arm puling herself up on an oak table, and she looked upon the scene before her. She began to remember, her days back in Equestria...
"PLEASE HELP ME!" Twilight screamed in agony while Sombra cackled with delight.
Sunset remembered how Twilight had reached out to her when they first met each other in this world. She had believed Twilight's kind and forgiving words were nothing but a hollow ruse, but after seeing the kind of emotional strength and goodwill the other student had shown to the people of this world...
Twilight's next call for help was an incoherent gargle, her life was fading fast.
Sunset made her choice.
With all her remaining will, she rose to her feet and grabbed the man she had served for so long. Pain wracked her body as the lighting struck her already wounded form, but she held on long enough to throw Sombra to his death, at long last ending the long life of the wicked tyrant.
Sunset Shimmer collapsed, the damage her body had endured at last catching up to her. Twilight rushed to her side, hefting the wayward student over her shoulder to carry her to safety.
Just then, alarms began to ring throughout the palace. The alarms that only rang in the event of major structural damage. All across the building, Sombra's minions rushed to evacuate. Sunset wiggled loose of Twilight's grip and fell to the floor. "Leave me." She wheezed out. "You will not be able to escape carrying me."
"Don't say that Sunset, I can still save you!" Twilight said frantically.
"Oh Twilight." Shimmer responded, closing her eyes wistfully as a content look spread across her face. "You already have."
With the faint glistening of tears coming into the corner of her eyes, Twilight stood up and began to run.
"Celestia... Forgive me."
The mirror began to twist and shine, and in a flash, Twilight Sparkle returned to the world of Equestria, an equine once more. Her waiting friends rushed to hug her, Pinkie Pie taking a moment to put out the camp fire she had set up in the mirror room. As she felt the warm grip of the group hug, Twilight opened her eyes. Across the room was the pale blue form of Sunset Shimmer, giving Twilight a peaceful smile.