Luna has a secret. A secret that everypony forgot. Even her.
Long ago, before Luna became the creature know as Nightmare Moon, and before Sombra and Discord were problems, her and Sombra were friends. Maybe more that that, actually. They were researching ancient alicorn magic, and some of it corrupted Sombra. Yet he and Luna still liked each other, and searched frantically for a cure.
But not every illness can be cured, and not everypony can be saved.
Also, it has sad elements, but there's more funny.
An indistinct shape flitted through the shadows of the Everfree Forest and into the castle of the two royal pony sisters. Once inside, it solidified into a pony wearing a dark cloak that seemed to render it invisible to all of the guards. The cloaked figure ascended the stone stairs silently, glad that the cloak also silenced all noises made by the pony. Not hesitating once, the pony made their way to Luna's tower with not a single guard noticing it. Once outside the door, a gray hoof reached out and pushed the door open, stepping into Luna's room.
At that moment, Luna was deep in nightdreams, remembering her childhood. She was instantly taken back to when she was a filly, and going to the Academy for Gifted Unicorns. That was before Celestia had grown up and turned it into something even better.
"Come on, take the cloak off. Nopony will notice. Please?" A gray foal begged her.
"No! I don't want to," Luna responded angrily, looking at the foal like he had grown another head.
"Why not?"
"Because I'm a freak!"
"No, you're not! I like you just the way you are, and I think you look fine."
"But that's just you. What about everypony else? They think I'm a freak! You've heard them. Remember the one who tried to pull my wings off?! He almost succeeded! I couldn't fly for days."
"Yes, I remember. But nopony can see us right here. Plus, they're all too busy showing off the spells that they just learned. Pleeeeease?"
"Fine," Luna said as she reluctantly removed her cloak, exposing her tiny wings. "There. Happy?"
"Yes," The gray foal said, nodding his head. Then he reached forward and started shaking her, saying her name over and over, but in a deeper tone of voice. "Luna, wake up!"
Luna's eyes shot open, and she leapt to her hooves, her horn aglow and ready to defend herself. She looked wildly around, searching for the source of the disturbance. Her eyes alit on a sheepish looking stallion who had his own, curved horn aglow as well. Upon seeing him, she relaxed slightly, letting the spell she had ready power down. "Sombra? What are you doing here? And why has your poor horn become curved?"
"Heh. Hi, Lulu. Nice to see you?" Sombra said nervously, backing away slightly. The hood of his cloak had fallen down, showing his unnaturally curved horn and fangs. Luna stepped forward, a sad expression on her face. "NO!" He said loudly, startling Luna. "Don't touch me. I don't want to make you sick too," He said, his voice taking on a softer, more caring tone.
"Can we fix it?" Luna asked in a slightly desperate tone. "There has to be a way we can fix it."
"No, love. I've tried. The darkness can't be removed. Once it affects you, you're done for. But that's why I'm here," He responded quickly, looking at her steadily. "Remember what we researched? The spell the alicorns came up with to save themselves? I think we can use that."
"How so? It utilizes the magic of two ponies to create a pony form, a... child of magic..." Her voice trailed of, the implications of what she was saying dawning on her suddenly. "Oh. But you can't seriously be considering that. The darkness in you... You'll transfer it."
"But I won't. I still have enough of my own power remaining to do this, Lulu. You have to trust me. Please. I'm dying anyway," Sombra said, begging Luna. "Please..."
"Very well. I shall do it," Luna responded reluctantly, not meeting Sombra's eyes. "But are you sure?"
"Yes."
And with that word, Luna and Sombra stepped forward and touched horns, channeling as much magic as they dared through them. Within seconds a glowing bubble had formed around them, sealing them off from the outside world. Outside the bubble, the contents of Luna's room were swept up in the winds created by the powerful magic. Inside though, it was the eye of the storm, with no wind whatsoever. Even Luna's waving mane was still.
Slowly, a blob of magic formed between their horns, growing larger by the second. Something vaguely pony shaped formed inside the blob as the spell continued. Meanwhile, outside of the bubble, Luna's belongings crashed around loudly, denting the walls, floor, and ceiling. The noises were so loud that they awoke Celestia from the deepest of sleeps. Rising from her bed, Celestia called upon the Royal Guard, leading them to Luna's room.
When she flung the doors open, Luna's head swiveled, briefly disrupting the spell. The Royal Guards were flung violently away by the winds, and only Celestia was left standing. Celestia could only stand and watch speechlessly as the spell progressed. She tried firing bolts of magic that would stop the spell, but the bubble only absorbed the blasts and channeled them into the growing blob of magic. Soon it was as big as a barrel. A shell of sorts begin forming around the blob, turning it into an egg.
Luna, believing that the spell was coming to a close, started to reduce the amount of magic that she was contributing. Celestia, though on the outside, knew that that was the wrong thing to do. She could see Sombra struggling, trying to maintain his darkness free magic, but failing as the darkness consumed him. The dark magic exploded in all of its terrible rage, sending Celestia flying backwards into a wall, promptly knocking her unconscious. Luna however, still linked to Sombra by magic, felt the dark magic seep into her, corrupting her as it had done to Sombra. Sombra was immediately teleported back to the Crystal Empire where he would become its tyrannical leader.
The egg however, was teleported to the crystalline caverns beneath Canterlot, where it would remain for a long thousand years, waiting...
Luna walked through the halls of Canterlot palace towards the library. The sun was rising, so Celestia wouldn't be bothering her for a while. That meant that she could search the archives freely. She nudged her way through the door and came face to face with... "Twilight Sparkle?! What art thou doing in my library?"
Twilight backed up, surprised by the sudden appearance of Luna. "I'm looking for a book, Princess. What else would I be doing in a library? It's not like I'm hiding anything. Heh."
"Sleeping, perhaps? You have book lines on your face," Luna replied, smothering a giggle.
"I what? Um, well, maybe? Byegottago!" Twilight said frantically, dashing out the open door with several books held in her magic.
Luna looked after her as she rushed down the hall, into the wall, and down another hallway. "Huh. That was... odd." Luna quickly glanced at the catalog, searching for anything concerning shadow ponies and dark magic. As she suspected, there was nothing. This would call for deeper searching. "LIBRARIAN!!!" Luna yelled, employing the Royal Canterlot Voice. "LIBRARIAN!!!"
"Yes, my Princess? How may I help you?" A pale brown mare with a stack of scrolls as her cutie mark peeked her head out from a pile of books, rubbing her ears. "What kind of book are you searching for?"
"Oh. Sorry, Scroll Rose. Do you have any books on shadow ponies, Sombra, that kind of thing? I would look for them, but I don't have much energy in the daytime," Luna said apologetically, trying to sound not super interested in case Scroll Rose reported back to Celestia.
"Okay, but, uh... do I want to know why?" Scroll Rose replied cautiously.
"Uh..." Luna froze mid-step on her way to the center of the library, not sure if there was a correct way to respond without arousing suspicion.
"Say no more," came the reply. "And I won't breath a word of this to Celestia. Who knows what her reaction would be?" Scroll Rose went to look up the location of the books that Luna had requested. She suspected that they were in Celestia's special restricted section, but hopefully they weren't.
Luna froze again. "If my sister found out?" A squeak similar to that of a mouse's came out of Luna's mouth. Her eyes had shrunk to pinpoints at the mere thought of it. Moments later, Luna was down in front of Scroll Rose's hooves, begging frantically. "Pleasepleaseplease don't tell her. She won't settle for banishing me to the moon. She'll just flat out plain kill me."
Somewhat surprised at Luna's extreme reaction, Scroll Rose promised that she would never, ever, ever say anything. Ever. Luna thanked her profusely, and went to find a comfy reading spot, snagging the first Daring Do book to read while she waited. Moments later, Luna was deep in the world of Daring Do, and Scroll Rose was deep in the world of thousand-year old messy book cataloging. Daring Do was having much more luck at finding things.
An hour or so later...
"FOUND IT!" Scroll Rose yelled triumphantly, emerging from a pile of books and catalog papers.
"Uh huh," Luna replied distractedly, not bothering to tear her eyes away from the book. Currently, Daring Do was trapped and fearing for her life. Luna worried that if she stopped reading now, it was being unfaithful to Daring Do, and then Daring would also have no motivation to escape.
Scroll Rose replied, irritated with Luna's sudden lack of interest. "Luna, you wanted a book, I found the book, and now you don't care. Stop reading Daring Do for five seconds AND LISTEN TO WHAT I'M SAYING!!!"
"And, done! I have successfully completed the part where Daring Do is trapped. I must read more," the Lunar Princess exclaimed triumphantly, ignoring Scroll Rose's audible facehoof. "This A. K. Yearling pony writes the most wonderfulest of stories. I must have all of them!"
"... Luna."
"What?"
"...Seriously?"
"What?"
"You sound like Celestia with cake."
"You know about my sister and her cake addiction? She keeps it very secret. I might have said something to the press once, though..." Luna meandered off into silence, contemplating if that was considered funny or not, and if Celestia knew that she was the one who had leaked the secret.
"Yes, Luna. All of Equestria knows about your sister and her cake. Trust me, you only confirmed what everypony in the kingdom thought," Scroll Rose replied, facehoofing again at the way Luna acted. "Back on topic, I found where the book is. It is in Celestia's secret and heavily guarded personal library. There is no way that you will be allowed in there. The only ponies that have access to the place are the other princesses. Not you."
"I don't have access?" Luna looked at Scroll Rose, clearly hurt by her sister's lack of trust.
"No, you don't. Celestia banished you from the place when you tried to blow it up after coming back from the moon." Scroll Rose delivered this statement in a matter of fact tone, ignoring Luna's anguished wails and drawn-out moans. "Now stop that and listen to me."
It continued.
"Stop. Now."
It still continued.
"LUNA! WOULD YOU JUST SHUT UP AND LISTEN TO ME!"
"Ok," came the meek reply from Luna. The librarian was in full rage mode, with her gaze steely and her face perfectly calm. Scroll Rose's voice was full of the same deadly calm that filled her face. No pony knew how to intimidate like a librarian.
"Now, there are a few ways to get the book," Scroll Rose begin, talking to Luna like she was a foal.
"Oh! Break in! That sounds like fun," Luna exclaimed, throwing her hooves up in celebration of her idea.
"Luna, no. Just...no. Instead of maturing while you were on the moon, you got younger, didn't you? Should I give you to Twilight so she can test you?"
Luna's head shook frantically.
"I would suggest asking one of the other princesses to get the book for you."
Luna pondered the idea for a moment, deciding whether it would work or not. Cadance wasn't going to be visiting anytime soon, due to her being pregnant, but Twilight was never too far away. That, and Twilight loved books. She would be easy enough to persuade. Just use the right spell.
"Oh, and Luna, no using spells to break in or control ponies' minds, ok? You are a princess, and should act accordingly."
As scroll Rose walked off, Luna thought she felt something strange, but quickly dismissed the feeling as being nothing more than her being nervousited*.