A young thestral opened her eyes. She had a starry-night mane over her shoulder in a simple braid, like Princess Luna’s but without the flow, and a similar coat. Her batlike eyes were the purest of sapphire blues. She turned her head to look out the west-facing window. Celestia’s sun was setting; Luna’s moon was rising. She twisted out of bed, belly-flopping onto the floor and bonking her head against the bedside table.
That woke her up. Her eyes shot open as she regained her sense of balance and stood up. Below her, she heard her mother call, “Constellation, did you just fall out of bed again?”
“Yes, Mother,” she replied through the cracks in the pine floorboards. “I apologize for the noise, Mother.” Constellation then stretched her wings, as per usual with her evening routine. It was a comforting thing that always soothed her nerves no matter the situation.
“Breakfast” was munched on in earnest. Mother had made special oatmeal to celebrate her daughter’s sixteenth birthday, and was planning on getting a cake later that night.
“Is Father still worried about Princess Luna?”
“Yes. She's gotten worse. Her mind may be sick, but she won't let anypony close enough to find out. What your father believes is happening is some kind of decent into madness or extreme jealousy.”
The two ate in silence for a few minutes. Then the night routines came around, being the most normal of waking hours - and the dullest. Constellation sat outside, mostly, soaking in the soft moonlight. But as dawn was supposed to come, something odd happened.
“Mother! The sun is eclipsed!” Constellation cried.
“The eclipse has not occurred for seven hundred years! Why…” The adult thestral’s eyes widened in horror as she saw a huge flash of light. It originated to the east. The Everfree Forest. “Get inside!”
“What's going on?” her daughter asked.
“It's happened. Just like we've always feared. Princess Luna has fallen to true darkness.”
Even though their residence was in Canterlot, the duel could be seen easily. The many thestrals in this section of the city locked up, closed everything. The entire East Canterlot area shut down in a matter of minutes as their patron princess fought her sister. From inside their locked houses, they watched the flashes light up the night.
“Sister! Don't make me do this!”
“Once your petty daylight is gone, the night will last forever!”
The white-coated alicorn cast her spell. It missed the black thing in blue armor formerly known as Luna by the smallest of margins. She regretted the shot. She wanted to take it all back, to bring her sister back…
Her enemy cast a dark blue laser. It smashed a hole in the castle roof below. Princess Celestia dodged a second attack, casting another spell of her own. It hit the black thing square in the chest armor. She recoiled and lost her wing strength for a second. But that was all Celestia needed to escape down into the throne room.
The dark mare followed. Celestia brushed the stone dust off her shoulder. Scrapes and bruises covered her body, and her wingpits were starting to blister. “Sister, come back to me…”
“I am not your sister. I am the Night. My name: Nightmare Moon.” Nightmare cast a spell that wrapped around Celestia, picking her up and preventing her escape. She was dragged closer to the evil alicorn. “Once you are dead, I will rule all of Equestria. And there is nothing you can do to stop me.”
Celestia countered the levitation spell, dropping to the rubble-covered ground. She teleported to the stairs leading to the dual throne. Golden magic encircled the trapdoor between the two. “Then you leave me no choice. I'm sorry.”
Nightmare Moon saw it coming and flew up above the ruined castle before Celestia could do her thing. The night was beautiful. Dark and sinister and peaceful. It was almost harmonic.
Celestia now had six gems held in her magic. The orange one, honesty. The pink one, kindness. The deep blue, almost deep purple one, generosity. The sky-blue one, laughter. The red one, loyalty. The rich violet one, magic. All six swirled around the princess as she hovered just above the destroyed ceiling.
They both cast simultaneously.
The golden magic of the Elements of Harmony collided with the dark blue of Nightmare Moon’s. A sphere of light, brighter than the sun had ever been, expanded and engulfed both alicorns.
The shockwave was incredible. It ripped trees, knocked down houses, and sent every little critter flying for miles. It was so powerful, in fact, it was felt all across the continent. Changeling hives quivered. Even the cursed, disappeared Crystal Empire was shaken to its core.
In the sphere, the two opponents sent every last ounce of magical energy in them at the other. The magical strain grew to literal astronomical levels. Then it imploded.
All the magic was sucked into the six special gems and released in one blast directed at Nightmare Moon. It smashed through her feeble shield and annihilated her armor. With a final, loud cry of denial, she was transformed into pieces of magic that coalesced on the moon. The image of a mare materialized on the right side of the moon as the bright magic faded away.
Princess Celestia looked around her for a few seconds, hoping her sister was nearby. But when her gaze went upwards, she realized where Luna had gone. Tears began to stream down her face as reality kicked in. Celestia landed sobbing.
A single scroll appeared at her hooves. Picking it up with her magic, she read its contents. It was a prophecy dictating the return of Nightmare Moon in one thousand years. A full millennium without her sister by her side.
In Canterlot, the thestral families had all gathered outside as the fight came to a close. It was anyone’s guess as to what would happen next. Would they all be arrested? Would riots break out, demanding that they, the guardians of the night, be removed from their homes and relocated? What even happened over the Everfree Forest? Questions and rumors were flying around faster than the mythical Sonic Rainboom.
Constellation and her mother were stuck in the midst of this commotion. Finally, the young mare had enough. “EVERYPONY, QUIET DOWN! Quiet Down!” When she had everyone’s attention, she continued. “We can’t do anything about what’s going on. Whatever happened has happened. What we need to do is to get on our game and make sure we can deal with what happens next.”
“And exactly why do you have the authority to tell us what to do, Constell?” one asked. He shoved his way through the crowd to reveal himself as a snobby high-born thestral. “You are a peasant, the bastard daughter of a guard. You are the lowest-born thestral here!”
Constellation rolled her eyes. “Two things. One, I’m not a bastard. If my history is correct, that’s what you are. Second, my father is a guard of Princess Luna. I don’t see your father protecting our rulers with his impractical fancypants sword.”
Whispers about a burn rippled through the crowd. The high-born frowned, baring his bat fangs in anger as he hissed. His eye slits narrowed more than they already were. He looked like he was going to back down. Then, without warning, his hoof collided with Constellation’s muzzle, breaking it in several places and knocking her down.