Celestia was probably the only pony alive who knew it, but the festival had once been her sister's birthday. It was possible that this piece of trivia was recorded in some forgotten dusty tome, to be unearthed every century or so and remarked upon by some scholar. A curiosity, really. She was sure that the ponies who celebrated in the Canterlot streets below her had no earthly idea, and nopony had ever asked her much about the holiday's origins. It certainly wasn't a subject she discussed at length with even her closest companions. All anypony knew was that on Nightmare Night, the Princess Celestia kept herself aloof from any festivities, cloistered alone in her private bedroom.
Perhaps they believe that I find it distasteful, Celestia thought, staring out her window at the bright city lights. Or that I consider their stories and superstitions foalish. Most of them consider Nightmare Moon a foal's tale, or some long-vanquished evil, never to be seen again on this Earth. She did her best not to keep herself secret to her subjects, to open her heart to her fellow ponies, but the truth of the matter was that her observance of her sister's birthday was a matter too personal for her to share.
There was very little that Celestia remembered clearly from the early years of her life, but the night of her sister's birth stood out like a beacon when she turned her thoughts to the distant past. The late Autumn storm that had pounded against the windows of her father's castle, the screams of her mother echoing through the halls. All of the fear and excitement on the faces of everypony around her, and her a mere foal, barely old enough to understand. How she had been allowed into her mother's room at last, and seen her mother's sleeping form in the crackling firelight. And there was her sister, curled in a blanket in her mother's forelegs, little more than a ball of blue fur. As Celestia had approached, the infant foal had turned, and with wonder, Celestia looked into Luna's eyes for the first time.
More than a millennium later, Celestia was looking at another face, the face that had appeared in the moon when Luna had been banished. It was not her sister's face, but the face of the monster she had become. It seemed to glare at her, to accuse her, to demand to know if she had the strength to proceed. The ordeal she faced tonight was one she had faced many times before, but still the thought of it gave her pause. She had stalled and dithered, lingering at her window, but she could put it off no longer. She gave one last glance to crowded streets, then closed the windows with a flicker of magic and turned to her business.
Celestia's usual evening rituals were observed. She removed the ornaments of her office, setting her crown on her beside table and gently slipping her hooves out of their golden plating. Another quick spell stilled the fire in the hearth, bringing the room into darkness. As her eyes adjusted to the dim moonlight, she gave her chambers one last look-over to make sure everything was in order, then settled herself between the sheets of her large, four-poster bed. The servants had orders not to disturb her.
Instead of letting herself drift off to sleep, Celestia closed her eyes and turned her attention to a spell—or rather, a complicated and interlocking series of spells, an arrangement that she had devised herself, that enclosed and protected her own mind. It was an impenetrable barrier that kept out any outside influence on her thoughts, and one of the side effects was that when she slept, she did not dream. For three hundred and sixty four days out of the year, her nights were filled with an undisturbed darkness. It pained her to do this, but for the sake of her sanity and her ability to function as a ruler, it was preferable to the alternative. Tonight, though, she could allow herself to face the turmoil that awaited in the world of her dreams.
Her horn pulsing with magic, Celestia slowly and deliberately unraveled the barrier. It was no simple task; she lay perfectly still, her entire mind focused on the complex magic she performed. There was no visible evidence of the spell, but in her minds eye she imagined it as a series of curtains, pulled back by silent stagehands. Beneath at all her her mind, her soul, everything that was her, and for this one night it was laid bare. She felt both free and extremely vulnerable. The spell undone, she inhaled deeply, releasing the breath she had been unaware of holding in.
Actually falling asleep was another matter entirely. Aware of what was coming, her body instinctively tensed. For several long minutes, all she could do was stare at the cloth canopy that hung in the darkness above her. It was an embarrassment, really. The ruler of all of Equestria, frigthened to fall asleep, like some foal who clung to its parents. But by controlling her breathing, by carefully relaxing every muscle in her body, she was able to drift into the murky world of slumber.
Celestia found herself in a place that was not a place, standing on nothing, in total silence. She was neither hot nor cold. There was no wind, or even a sense of air around her. There was no apparent source of light, yet in the odd logic of a dream, she could still see the void stretching forever around her. She knew she would not be alone for long, though. A terrible beating of wings came thundering out of the darkness, and in a moment's time, a massive figure rose out to loom over her. A thousand times larger than life, every movement an earthquake, every breath a cacophony, Nightmare Moon stood grinning down at Celestia.
“You pitiful little foal!” she roared. “You dare to come before Nightmare Moon?!”
Though she had been long banished and her power weakened, though she was not even aware of what was going on, the Princess of Dreams could still enter her realm. Wherever the creature that called itself Nightmare Moon went, Pony's dreams became visions of horror, and over the years superstition had caused bad dreams to become synonymous with the name of Nightmare. But oddly enough, on the very night that ponies payed homage to this figure of legend, dreamers across Equestria were spared from any unpleasantness, as the monster was drawn to Celestia's unguarded mind.
“I'm sorry, sister. Please forgive me,” said Celestia. Nightmare Moon did not seem to hear. The beast's massive black wings spread wide, and as they came down to enfold Celestia, the nightmare began.
A multitude of terrors overcame Celestia. Jumbled scenes flashed before her eyes, there and gone in an instant, and filled with snarling monsters and horrible destruction. A roaring dragon spat flames, which became a torrent of skittering insects that swarmed over her, which became a howling storm that battered her body. The faces of ponies she had known, from years and decades and centuries ago, rose up before her, sometimes filled with anger, other times with fear or sorrow, shouting obscenities, begging for mercy, or simply staring blankly.
The visions were accompanied by sounds, not always connected to what they showed. A tumult of screams and roars and cries came at her from every direction. She heard her mother's voice, sobbing in agony. She heard the laughter of foes she had long ago vanquished. Her body, too, was subject to the flood of nightmares. One moment she was falling, another she was drowning, another she was trapped in stone. The dream was an assault to all of her senses, and it went on relentlessly. Time seemed to have no meaning.
It had no real effect on her. Celestia let it all happen, took it all in. This was not the difficult part. In a way, it failed to be frightening at all. There was no purpose, no calculation, no real attempt to build up fear in its subject's mind. Each nightmare transformed into a new one in a moment, too quickly to to be reacted to. It was the senseless dream of a pony who had long ago gone mad. All she saw when she looked, all she heard when she listened, was the pain that her sister was enduring, the pain that she knew she had caused. She squeezed her eyes shut.
“I'm sorry,” she whispered, and the noise around her was nothing compared to the sound of her own voice. “I'm sorry, I'm sorry, I'm so, so, sorry.”
“Sorry for what?”
Celestia opened her eyes. In an instant, all the sounds fell away, and although she knew what she would see, as she had seen every year since she had banished her sister, it still caused her the same painful jolt in her chest. She was once again hanging in darkness, but this time it was not Nightmare Moon who stood before her. Nor was it Princess Luna as Celestia had known her for centuries, regal and proud and brooding. It was the Luna that only Celestia remembered, the slightly chubby little filly with the squeaky voice who had annoyed her older sister by trailing her endlessly through the castle corridors. With an awkward shuffling of her hooves and a self-conscious toss of her mane, Luna gave her older sister a nervous smile.
“Hello, Luna,” said Celestia, managing a small smile herself. Her body still shook from her experience, and she did not need to look down to know that the form she wore was not the one that slept in her chambers in Canterlot, but that of the gangly and awkward filly that she still thought of herself as after all these centuries.
“Hi, Celly!” said Luna, her uncertain smile growing into a grin. “What did you say you're sorry for?”
“For not getting you a present,” said Celestia. She stepped forward, moving effortlessly through the void, and pulled her sister into a hug. “Happy birthday, Luna.”
Luna pulled back, her eyes widening in surprise. “It's my birthday? Oh my goodness, I didn't even remember!”
“It's okay,” said Celestia. “I remembered for you.”
“I, um...” Luna looked around, confusion furrowing her brow. “I'm having a hard time remembering anything, actually. Am I dreaming?”
“Yes, Luna,” said Celestia with a nod. “You're dreaming.”
“Oh.” Luna turned back, and squinted at Celestia. “But you're really here, right?”
“I am. You're the Princess of the Night, remember? You can enter anypony's dreams you like. And tonight, you've decided to come into mine. Thank you.”
“I'm... the Princess? It's all so blurry. I feel like,” Luna frowned. “I feel like I've been asleep for a long time.”
Celestia took in a sharp breath. Try as she might, she couldn't bring herself to lie. Not here, not to Luna. “You have been. I'm sorry.”
Luna's frown deepened. “Am I okay?”
“No.” Celestia struggled to keep her expression neutral. “No, you're not. And I'm sorry, I'm so sorry. But I'm working on it, I promise. I'm going to fix it, and we'll see each other again soon. I promise you Luna, I promise you with all my heart.”
Fear crept into Luna's features. “I feel like I've done something wrong. Like I've hurt you somehow. I'm trying so hard to remember, but it won't come back to me. What happened, Celly?”
“It's alright,” said Celestia, pulling Luna back into the embrace. “I forgive you. I love you, Luna. Please try to remember that. I forgive, and I love you, and I want so much for you to come home.”
“Well duh,” came Luna's voice in her ear. “We're sisters, aren't we? What could ever come between us?”
Celestia found herself unable to respond. She could only pull her sister closer and shut her eyes tightly.
When Celestia awoke, her cheeks and pillow were damp with tears, and her sheets soaked with sweat. She wiped them away as she rose shakily from her bed. Although she had slept the whole night, she was thoroughly exhausted. But the rhythm of night and day called to her, an invisible string that pulled at her mind, as familiar as her own heartbeat. She stepped to the window to perform her celestial duty, and her horn glowed as the sun rose over the horizon.
She kept her magic going, setting to work patiently reconstructing the spell that kept her separated from her sister. Another year of empty nights lay before her. And there would be another after that. And another, and another, and another. All that was keeping her from total despair was the knowledge she was slowly building up, the bits and pieces she was gather from books on ancient prophecies and the Elements of Harmony. It was slow going, the work of centuries, but slowly, surely, a plan was beginning to take form.
“I'm working on it, Luna,” Celestia whispered to herself as the first rays of the morning crept through her window. “I promise.”