(Or the sound the sun makes when it goes out)

In the late afternoon, between shafts of golden light that cut through still and dusty air, Celestia surveyed the empty Solar Court through equally empty eyes. Habit had brought her there, the muscle memory of centuries dragging her limbs without conscious thought through the corridors of the palace, on a route that had long ago become so ingrained in her mind that she might be able to draw every nook and cranny and crack and carpet of it from memory.

Why was she there? The day had been set aside on her calendar for months. Nopony had expected her to be there – not even the usual hopefuls who turned up on national holidays, eager for the chance to catch her unawares, as if that might help whatever petition they had brought. She had no reason to be there, on her throne, waiting for courtiers who would not arrive. They had been told to stay away.

The light of the sun inched further across the empty floor, lighting the white marble gold and blue, and green and purple, and every other colour contained within the tall, slender windows that depicted events from Equestria's history. All except one.

It had been so long.

"A thousand years," she announced to the room at large. Her voice echoed strangely without the ever-present mass of ponies to absorb it. The noise served to wake her mind from whatever fuge it had entered. Celestia looked about herself, taking in the still air and the golden light. Light that had already started to take on a somewhat strained quality to her arcane sight.

Carefully, one limb at a time, Celestia tore herself from the comfortable, familiar throne and descended the dais that served to isolate her from the world. She took one last look at the throne and bowed her head. With a gentle touch of her magic, the crown she had worn for the last millennium slipped from her head and settled on the floor at her hooves. She turned and walked toward the great doors of the throne room.

Those few servants that remained in the palace watched warily as she stalked toward the western wing. If they noticed the absence of the crown on her head, they said nothing, and in a perverse way that fact pleased Celestia. She had always prided herself on the discretion of her staff, especially in those days and months when she allowed herself to partake of what might be considered less than savoury pursuits. No scandal had ever erupted, not even that year she had spent seducing every pony that crossed her path. Or perhaps their discretion was found in not informing her of the scandal...

But that was the talk of paranoia, and she was not a paranoid princess. Raising her snout just a little, Celestia forged onward toward the west and her sister's domain.

Luna had always insisted on maintaining her apartments to the west of Celestia's living accommodation, and that habit had not been broken by her lunar sojourn. In the west, she said, the sun would set itself and the dark of night reined the day for a time. It was her idea of a joke.

The corridors around Luna's private rooms were deserted and silent, shrouded in magic that was as novel to the world as it was ancient, its like not seen for centuries. Even at this remove its crackling energy tingled in Celestia's spine and out to the tips of her outstretched wings, tantalising her body in ways she had forgotten were possible.

A door opened as she approached, admitting Celestia to the dimly lit corridor beyond. She paused beyond the threshold; the door closed behind her in near silence, trapping her in that place as surely as any cage.

"Luna?"

There was no answer to her call. Perhaps Luna had fallen asleep—but the magic was still strong, still growing. She wouldn't be asleep for this. Steeling herself, Celestia pushed through the curtains that bound the corridor and into the space beyond.

Bright sunlight filled the room, flowing through a bank of slender, gothic-styled windows that filled an entire wall. A fire pit of white marble rose from a silvery granite floor. Any other details of the room seemed to slip from Celestia's mind the moment she turned from them, leaving only the faintest impressions of form and colour. Black steel, velvety red, something that might have been a tapestry or a curtain across another door. No fire burned in the pit, and when she looked up at the window she saw the sun hanging beneath the moon in a bright afternoon sky.

"You came," said a voice. Luna's voice, she thought. Celestia turned from the sky, but no matter how hard she looked, she couldn't find her sister.

"Did I have a choice?"

Shadows flowed across the walls, coalescing around a dark recess in the wall. She blinked, and the recess was a doorway, and Luna was stepping out into the light.

"We always have a choice, sister," she said, with just the hint of a smile. Her coat was dark, far darker than usual. "Or so I had believed. You have come freely?"

"I have. But Luna, it will terrify them. They haven't seen anything like this for centuries."

Luna's steps took her in a circle around Celestia. "Your careful regimenting of the sky saw to that, sister. The moon fixed to the night, the seasons and years tied to your sun," she said as she passed by Celestia's rear, and paused a moment to smile at her mark. "The wanderers forgotten, the cycles forgotten. The wax and the wane, approach and retreat..."

"I had no choice."

Luna stood before her again, though Celestia couldn't quite recall when she had reached that point. "Perhaps while I was gone, that is true. Yet now I am here once again."

"And I," Celestia replied quietly. She raised a hoof, moved a step toward her sister. "Luna, it's been a long time. The world changes."

"You hint of social mores and reputation ." Luna snorted. "I am the epitome of change, Celestia. I know that flux is the norm, but change must be wrought upon eternal foundations or it will come to nothing. The moon wanders night and day, changes her countenance, and brings inspiration and insanity in equal measure, but she is always the moon. She exists, no matter how her face might alter."

"They have forgotten."

"Then let them remember! And let you remember too," Luna said. She leaned toward Celestia, raising her mouth close to Celestia's ear as she spoke. "You want this, you need this, sister. I know of your dalliances and your private scandals, how you sate your passions and seek oblivion between the legs of others. Oh yes, I know. The palace staff are not as discreet as you wish to believe, when their bellies are filled with food and mead and their minds tantalised by visions and dreams."

So much for that, Celestia thought, lowering her head. "It's true that I desired... but ponies cannot accept such things! It would be abhorrent to them."

"Abhorrent." Luna stepped away, turning to look out at the moon. By now few could have failed to notice its proximity to the sun. She imagined Twilight frantically studying the event, rushing from book to book as she tried to understand the processes involved. "They accepted it before. They celebrated it, with horns blown across the land and feasts in our honour!" Luna took a graceful leap to the sill beneath the window and turned, spreading her wings so that the sun rested over one and the slender crescent of the moon over the other. "Every new moon they would call out and feast and celebrate, and we would make love beneath the darkening sky. Every month we would renew the bonds of our spheres before the assembled tribes! You revelled in it, sister! You rejoiced and you were taken by the darkness, and born again to new light!"

With panting breath that ghosted white in the room, despite the warmth, Luna looked down at Celestia. Her eyes shone and her cheeks glowed, and steam rose from her back in the sun's bright heat. Celestia was panting too, despite herself, as ancient magic flowed between them. She tried to blink, but found her eyelids langorous and slow; tried to speak, but found her tongue thick and her mouth slack. She could only groan beneath the pent up desire of a thousand years.

"Remember it," Luna called, as she stepped down to the floor.

"Remember," she said as she strode toward Celestia's shivering form.

"Remember," she whispered, as her face drew close to Celestia's own.

And then her breath was against Celestia's cheek, hot and moist as a summer's night. Celestia's lips parted; she closed her eyes as Luna pressed forward against her. They kissed, and as the sun was put out and the moon cast her shadow across the earth, Celestia fell into the black embrace of Luna's wings, and lost herself to oblivion.

Author's Notes:

Reposted from the princest collab (just in case; or for the sake of completeness. Who knows?)