High on the weathered battlements of Canterlot, Celestia looked to her guards, to her sister, to her young friends. In each pair of eyes she saw the same fear, that dread that comes with knowledge of the inevitable, that today would spell the end of Equestria. Celestia's mask of the omnipotent ruler was discarded, in its place she wore a face not seen by mortals in centuries-- one of grim resignation.
Not five days ago had the disaster begun. To help usher in the new era beginning the corregnum with Twilight Sparkle, the youngest princess had a request. To reclaim the old Palace of the Royal Pony Sisters, and finally erase the memory of darkness and strife that was that blasted, twisted place. In clearing the Everfree, the newly redeemed Discord had been leading the effort to reclaim the old palace from the chaotic magic that stained the region.
They had cut a path to the old palace grounds, with Discord gleefully finding his mastery of chaos magic indispensible in opposing the monsters, beasts, and ghosts of that desecrated place. Manticores turned to kittens, hydras to snakes, and pyric salamanders became newts before his magic, and progress was smooth. With high hopes the princesses watched the project from afar, eager to see the day when they alone would remember what evils had transpired there. To let those memories fall to the sands of time.
But, it seemed, not all memories were content to stay dead.
When the expedition reached the bowels of the ruined keep, something awoke - and Discord was taken. The Nightmare was back.
In less than a week, Equestria, from Vanhoover to Manehattan and even so far out as the Griffon kingdoms, was swallowed by the black madness. Equestria was unrecognizable, and the casualties incalculable. The Guard and its princesses were beaten back, and their pocket of sanity and refuge shrank each day. And now, with legions of the corrupted alongside new nightmares from the void outside Equestria, the Nightmare besieged Canterlot- where the Royal Guard stood to make a final stand. But there were too few, and everypony knew it.
Captain Shining Armor, along with every able-bodied unicorn he could muster, struggled to maintain the shield that stood between them and oblivion, or worse. Dozens of them had already passed out in exhaustion, and those remaining shook and sweated through pain and nausea from long effort. For ten hours they held back the invasion, a miracle in itself. But already cracks were showing, and their respite was soon to be at an end. The shield would fall, and already the skies darkened with the flights of eldritch horrors and the corrupted.
Twilight Sparkle asked the question on everyone's minds, "P-princess, what should we do when the shield falls?" With a morose glance toward her sister, Celestia gave the only answer she could.
"We die with some honor, in the hope that all is not truly lost. Tend to the pony next to you, see that they get just one more moment of precious life, because in that moment, and the one after that, each one of us gets just one more chance at a miracle." As if to punctuate her statement, the southern skies grew nearly black, and the shield wavered and fell in a flash of brilliance. But still worse was the silence, so quiet a pony could hear their own heart beat. Counting down their last minutes.
The sound of leathery wings from above disturbed the muted panic, and an emerald dragon of some dozen pony-lengths crashed down on the stone of the castle wall amongst them.
"Lightbringer, Mistress of Night, your majesties. The Great Wyrm sends word. In days long past, an alliance existed between ponies and dragons. Our two great peoples fought and died against chaos. We have come to honor that allegiance; we are proud to stand beside ponies once more."
The darkened skies to the south, now in clear view without the shield, thronged with dragons. Hundreds, maybe thousands, from whelps to wyrms, a sight that on any other day would cause panic and despair, provided but a glimmer of hope. They swooped and soared, gouts of dragonfire striking any that sortied too close, or fled too slowly. They would be upon the castle in minutes.
The draconic emissary threw the princesses a predatory grin, "Let us cleanse this world in Magic and Flame."
Luna looked to Twilight, and gave her a wink. "No regrets, sister." She spoke, but only for Twilight's benefit.
A solemn nod was Celestia's reply, and the colors washed from her mane, replaced with the burning fury of her celestial charge as she was consumed. The remaining ponies leapt back at the waves of heat and falling cinders.
Celestia Lightbringer, Sol Invictus, leapt from the battlements, a blazing beacon, and spread her wings to meet the enemy. Luna, for her part, weaved the shadows cast by her sister about them all, and around each standing pony coalesced armor of shifting shadow, upon every wall that once was bare now stood an army of shades, half-pony, half-shadow. And at the center Luna cackled, a deep laugh all too familiar to the Elements, and she too became swathed in stars and shadow. Her magic done, a now black, armored avatar of night leapt into the skies after her sister.
Stunned, Twilight, too, spread her wings, but no elder magic came to her. She paused to glance back at her friends, her friends! With a moment of focus, from deep in their chests, their hearts and elements glowed. Brighter and brighter, blindingly so and then still brighter. Soon even Celestia wreathed in flame was indistinct, and just as abruptly as it had started, the light went out and a flash of rainboom and rainbow contrail could be seen emanating from a tiny lavender bullet launched from the ramparts after the elder princesses.
The voices of the princesses of ponykind rang out across the city one last time, "Hold fast my little ponies, by the Light we shall not falter!"
"By the Night we give no ground!"
"BUUUUUUUUUUUUUUCK YEEEEEAH!"
And the Battle of Canterlot was underway.