Once upon a time, in the magical land of Equestria, there were two regal sisters who ruled together and created harmony for all the land. To do this, the eldest used her unicorn powers to raise the sun at dawn. The younger brought out the moon to begin the night. Thus, the two sisters maintained balance for their kingdom and their subjects, all the different types of ponies.
But as time went on, the elder sister grew proud. The ponies relished and played in the day she brought forth, but slept through her sister’s beautiful night. One fateful day, the elder unicorn refused to lower the sun to make way for twilight.
The younger sister tried to reason with her, but the arrogance in the elder's heart had transformed her into a wicked tyrant, Corona Blaze. She vowed that she would engulf the land in eternal day.
Reluctantly, the younger sister harnessed the most powerful magic known to ponydom: the Elements of Harmony. Using their magic, she defeated her older sister and banished her permanently from Equestria.
The younger sister took on responsibility for both sun and moon, and harmony has been maintained in Equestria for generations since.
Sunset Shimmer slammed the book shut and rolled the kinks from her neck. Now that she was done with the stupid storybook, she could get back to her real studies. A twitch of her horn summoned a burst of cyan magic that brushed the old tome from her desk. Before it thumped to the ground, she was already finishing a second spell to levitate a glossy new treatise into its place. She delved eagerly into the arcane diagrams and spell formulae.
It was annoying, how Princess Luna kept diluting Sunset Shimmer’s lessons with these silly detours into mythology and history and other useless subjects. Still, she made sure to learn the material, and learn it well. The princess was the wisest, most powerful pony in Equestria, and putting up with a few worthless distractions was a small price to pay for being her personal student. Sunset Shimmer had access to the most advanced materials, enough clout to speak with the foremost experts in every field, and personal instruction from the greatest magician who ever lived.
And… it was nice, sometimes, to have her around. Princess Luna was the only pony who could keep up with Sunset Shimmer as she bounced from topic to topic, drawing connections between fundamental aura theory and recent breakthoughs in applied morphology, or speculating on how horn length affected astral attunement. Even with the so-called experts, Sunset Shimmer needed to slow down and spell out every obvious detail before they could understand. With Princess Luna, she could just… talk.
And there was so much to talk about. The treatise described a variant classification of thaumaturgical interdependencies, but the authors barely seemed to understand their own ideas. Sunset Shimmer flipped back to the start and read through again, envisioning the new model. She slowly added pieces to her mental diagram until she had a sprawling three-tiered model of the whole thing. Yes, this approach was fascinating. She couldn’t wait to tell the princess. It suggested a connection between—
“There you are, Sunset!” The high-pitched voice tore her concentration to shreds. Her model vanished like dew in the morning sun, replaced by the library’s mahogany floors and clerestory windows and, incongruously, half a dozen giggling ponies. It was Twinkleshine, of course, with her gaggle of empty-headed hangers-on. “Moondancer is having a little get-together in the west castle courtyard,” she went on. “You wanna come?”
This was why they had interrupted her? Did they even know what a library was for? “I have better things to do than socialize,” she said through clenched teeth. “I’m going to re-read Advanced Elemental Magical Practices and then practice for my advanced summoning spells exam.” Princess Luna planned to administer the exam personally, and Sunset Shimmer’s performance would be perfect. She levitated the books into her panniers and slung them across her back. “Maybe you can help me, though. Do you know a good place where a pony can get some studying done? A quiet place?”
Glowering, Twinkleshine stepped forward and opened her mouth. Before the tirade could start, Sunset Shimmer fixed the pony with her best glare, channeling the intensity that would have been devoted to her studies. Twinkleshine faltered. “Maybe the Vesper Tower,” she mumbled.
“Perfect!” Sunset Shimmer said through a false smile. “And don’t worry, girls. When the princess asks what the city’s most diligent students are up to, I’ll be sure to tell her all about your little party.” She turned her back and walked away.
“Does that pony do anything except study?” Twinkleshine said in a whisper that Sunset Shimmer was pretty sure she was meant to overhear. “I think she's more interested in books than friends.”
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The rainbow lights faded, leaving Corona Blaze sprawled in a heap among shattered bits of armor. The largest piece was the great ivory helm, a jagged crack now running along its dome. The pony seemed smaller, somehow, and her burning mane had faded to a softer glow, pink and turquoise and cobalt.
As the Elements’ power dissipated, the sense of wholeness they brought dissolved as well. Sunset Shimmer tried to hold on to the feeling, but it slipped from her grasp like dust through a sieve. She felt empty, alone, worse than she ever had before… no, she felt exactly the way she had always felt before she came to Ponyville. She’d just never known until now that there were other ways to feel.
“We did it!” came Pinkie Pie’s shriek from behind her. “We totally saved Equestria!” Sunset grit her teeth. Source of limitless magical power or no, these ponies could get annoying.
“Gee, Sunset,” said Applejack, “I thought you were just spoutin' a lot of hooey, but I reckon we really do represent the elements of friendship.” She laid a hoof on Sunset’s withers, and for half a heartbeat, the sense of wholeness flared again in her breast. Okay, maybe there was something to this friendship thing after all.
“Indeed you do,” came Princess Luna’s voice.
Sunset Shimmer whirled. Princess Luna walked soundlessly across the castle floor, calm and collected as though this were a perfectly ordinary visit.
“Princess.” Sunset’s eyes tensed. “You never told me the story was real.”
The princess half-smiled. “You never would have believed me. Instead I told you to focus your studies on friendship. I knew you had enough magic, and I knew you had the strength to let friendship into your heart.” She turned to Corona Blaze. “Now if only another will, as well. Princess Celestia.”
Corona Blaze lifted her head and let out a little wordless gasp.
“It has been a thousand years since I have seen you like this,” said Princess Luna. “It’s time to put our differences behind us. We were meant to lead together, big sister. Will you accept my friendship?”
Corona Blaze—Celestia—struggled up to her forelegs. She stared at Princess Luna with trembling eyes, half-turned away as though she were about to bolt. Finally her gaze softened. “I—”
There was a flash of rose-colored light, and a purple unicorn appeared at Celestia’s side. Sunset Shimmer blinked. A teleport like that took more magic than any three of her teachers together could muster. She could barely cast the spell herself. Who was this unicorn?
Sunset had only a moment to wonder before the unicorn launched a ray of raw magic at Princess Luna. The princess conjured a cobalt shield, stopping the attack harmlessly. Sunset Shimmer gaped, stunned by the unicorn’s sacrilege.
“Twilight Sparkle,” said Princess Luna. “I owe you an apology.”
Twilight Sparkle stood before Celestia, shielding her with her body. “You leave her alone.”
Rainbow Dash streaked forward, the Element of Loyalty glowing crimson at her throat. “Out of the way, pipsqueak.”
“Enough, Rainbow Dash,” said Princess Luna. “Twilight, there’s no need for this. We’ve stopped fighting. Celestia is coming home.”
“No.” Twilight Sparkle looked to Celestia. “No, it’s not over. You can win another day. You’re better than this. You’re better than her.”
Celestia’s eyes darted from one to the other. “I,” she said, “I don’t… I need to think…”
“I’ve missed you so much,” said Princess Luna. “Please.”
Twilight Sparkle took a breath. “I believe in you.”
Slowly, eyes squeezed shut, Celestia reached out a hoof. She took the cracked ivory helm and clutched it to her breast. Twilight Sparkle stepped closer, her face a rictus grin. A flash of rose, and they were gone.
It was Rarity who broke the stillness. “What in Equestria was that?”
“My two greatest failures,” said Princess Luna.