Author's Note
Often times, fanart is what sparks a fic for me. The inspiration for this one is complicated, but this was a large part of it.
I beg of you your indulgence, everypony. This will be a labor of love, and I ask that you be patient with me. All my heart and soul is going into this story. 
Prologue
The Sisters' Equestria had never known war. The fires of battle had never raked her lands, nor had the rains of loss ever fallen upon her people.
Their leader had seen to that.
Celestia, Princess of Equestria, Royal Celestial Sister and Goddess of the Sun, elder of Luna, the Moon Goddess. Ethereal beings placed upon the land to watch over ponykind.
That was the myth she had allowed Starswirl to perpetuate eons ago. Before the young alicorn had stepped into her regal shoes and assumed the throne in the Everfree Forest so many years ago, before she had been forced to banish her only remaining family member to a cold, dead orb hovering the void, Celestia had been a filly.
And she had been afraid.
"My Queen..." Her father stepped back from the window of the throne room, his expression one of finality. "It is time."
"I know," her mother said softly. Celestia clung fiercely to her leg, burying her muzzle in the constantly shifting colors of her coat.
"This realm will not survive another night of Chaos," her father continued, the strong sound of his hooves on the stone of the floor signaling his approach. "We must-"
"I know," her mother snapped harshly. Her voice caught in her throat. She lowered her head to embrace Celestia with her neck, the ethereal flow of her mane mingling with the soft cottony pink of her daughter's.
"Child," her father said, kneeling to look her in the eye. His dark complexion had always captivated her. When she stared into his eyes, she could see forever. The promise of eternity glimmered in the void of his mane. "We love you..."
"Father," she replied. He rose before his eldest could see the tears in his eyes. Her mother nuzzled Celestia again, urgently.
" 'Tia," she whispered lovingly. Celestia looked up at her mother. "Remember... how we raised you."
"Mother?" Celestia asked, her heart suddenly nervous.
"Take care of your sister, my Eldest. You are my light," her father said, his voice thick.
Her mother stepped away from her daughter and went to join her husband. Celestia started to run after her mother, but a strong force stopped her. Her mother's horn was illuminated, the soft glow catching the tears trailing down her cheeks.
"Look after the kingdom, Celestia." Her father's voice was stern. "Until Luna is ready, you must perform both your tasks."
Her mother's magic kept a gentle but firm grip on Celestia. She squirmed in protest. This was not right. The flashing lights and loud noises had been going on outside for weeks, and now Mother and Father were leaving her.
The celestial pair took wing, a gloriously frightening sight. Light and void spiraled upward toward the opening in the throne room's ceiling until they vanished from view. Celestia watched with a growing sense of panic.
"Why...?" she asked the emptiness, staring up at the opening where her parents had disappeared.
The horrible sounds outside continued.
Celestia dashed to the window, clambering up the stone ledge to place her hooves on the sill.
Her face fell.
A sheet of magic sheared an earth pony from neck to tail of its skin, leaving a steaming mannequin of musculature twitching in a morbid dance. The unicorn who had cast the spell threw her head back and laughed, her eyes blazing with madness.
A griffin took a spear to the chest, blood gushing like a fountain from the arterial wound.
A dragon writhed upon the ground, consumed from within by its own fire.
Above it all, in the empty twilight sky, hovered her parents. Celestia watched with wide eyes as they whirled about each other, the very air seeming to melt between them.
The explosion of light was silent and painful.
Celestia fell from the window, screaming at the burning sensation that consumed her flanks. From the nursery, she could hear her sister foal shrieking in a similar vein. It was as if hot metal was being held to her skin. Swallowing her discomfort, Celestia hauled herself to her hooves and peered out.
Hovering low in the sky, the dark orb was rimming by a halo of light. Celestia watched as the darkness slowly separated from the brightness, and the two circles arced across the heavens to sink below opposite ends of the horizon.
Her flanks throbbed.
The horrors outside had ceased. There was no evidence of destruction, or of death. Only the forest and castle grounds lay peacefully beneath the now dark sky, in which twinkled thousands of tiny white lights.
Stars?
Luna's wails grew louder and more urgent. The sound of her hooves on the stone of the throne room echoed louder than any sound she'd heard before. And she knew.
Her mother and father were gone.
***
Equestria: Present Day
"Good moonset, my sister."
"Blessed sunrise."
The two alicorns stood facing each other on the rocky outcrop the overlooked the field. In the distance, Canterlot's spires rose in the sky. Below, the camp was beginning to stir, cooking fires sparking to life, soldiers rising and armor clinking as Luna's army prepared for the day. Celestia watched her sister descend the incline.
"Why have you come here?" Luna demanded, her expression impassive. "Do you surrender to my demands?"
"Luna..." Celestia's tone was pleading. "Can we discuss this?"
"You can raise the sun from Canterlot Castle," Luna said sharply, narrowing her eyes. "To broach my perimeter only invites an attack upon your person."
"Equestria belongs to us both," Celestia reminded her.
"I am done with your 'negotiating'," Luna snapped, stomping a forehoof. "And you made who rules Equestria clear the day you banished me!"
Her words were like crystal shards, and Celestia felt her heart ache with each syllable. Such poison from her sister's lips. Such venom in her tone. Could she not see reason? So young, so little, she could not remember their mother and father...
"You have until moonrise," Luna continued, taking care not to brush Celestia with any part of her being as she passed her sister. "If you do not concede... I press forward."
"Luna!" Celestia called. Her plea fell upon deaf ears as the Moon Goddess walked away. Celestia let her head hang. Battle or no, she was already defeated.
With tears in her eyes, the Royal Sister raised the sun.
It rained in Equestria that morning.
