A/N: This is more or less an idea about the final days of Equestria. I'm quite fond of Princess Celestia, but nothing good can last forever.
The Last Sunset
The sky over Equestria burned. Heat waves shimmered over cracked and dry earth under a red, cloudless sky. Even the onset of blessed night did little to cool the scorched landscape. Atop the tallest tower in what remained of Canterlot's castle, Princess Celestia watched the horizon flicker like coals in a banked hearth, her white coat streaked orange and red from the day's dying light.
"Celestia, please," a hoarse voice said behind her. "You must stop this."
Princess Luna was haggard, her dusty midnight blue coat ragged. Her mane, alive with glittering stars and galaxies, was dim, as if the night sky were being washed away by encroaching dawn. Celestia continued to stare at the fading sun.
She hadn't spoken to anyone in years. Even her once-expressive eyes had grown blank. After that, the sun began to grow, edging closer to the world which relied upon it for sustenance. The weather had grown violent, so unpredictable that countless lives were lost. Crops began to wither and the sky turned from pale blue to a sickly orange. A few ponies yet survived, eking out an existence under the moon's silvery glow, but there wasn't much their other princess could do for them.
"Sister, why won't you speak to me?" Luna whispered.
A blue-feathered wing brushed against Celestia's side and she turned. The face that met Luna's grief-filled eyes was empty. The princess of night swallowed, fighting tears. She had known what she would see: it was the same face she had seen every night and every day for far too long. She braced her hooves against the dry stones and bent her head.
"Celestia, I'm sorry."
With all her might, Luna threw herself forward, magic writhing around her horn. The obsidian crown fell from her mane, clattering against the stones as her horn sank deep through bone and flesh. She felt the heart which had beat strong for millennia spasm and shudder while tears coursed down her cheeks.
Celestia's wings flared out in surprise, as if she might take flight from the inevitable. Luna's magic flared through her veins, the only power great enough to sooth her body's internal furnace. The sun princess made a retching sound and coughed, blood as red as the sky dribbling down her white chin.
Shivering, Luna wrenched herself free, her sister's blood trickling in dark rivulets down her horn and into her star-sprinkled mane. Celestia still stood, eyes wide and shocked, wings arced behind her. For the first time in years there was a soul peering out of her.
"L-Luna..." she rasped, "th-thank..."
A flicker of relief shone in Celestia's eyes before they rolled back and she sank to her knees. Her glorious wings fell slowly to her sides and her head sank, gravity pulling weary eyelids shut for the last time. The sun princess of Equestria could have been sleeping save for the blood pooling between her forelegs.
Luna touched her red-stained horn to her sister's one last time. With a sob, she turned and watched the moon rise, big and bright. Slowly, the world began to cool.