=====// \=====

Celestia bestowed a glowing smile on the happy couple in front of her as she declared in a ringing voice, “Princess Cadence and Shining Armour, it is my great pleasure to pronounce you-”

The doors at the end of the grand throne room flew open with a crash to reveal the dishevelled figure of Twilight Sparkle. “Stop!”

It was all she could do not to huff in exasperation as the guests' confused voices filled the air and to one side, Rarity clapped a hoof to her head. In front of her, poor Cadence summed it all up and began to cry in sheer frustration.

“Ugh! Why does she have to be so possessive of her brother? Why does she have to ruin my special day?”

“Because it's not your special day,” came a loud voice, full of steely resolve, “It's mine!”

A pastel-pink pony slid to a halt beside Twilight in the aisle with her blue-banded wings spread in threat. A pony with a horn poking through a familiar three-coloured mane and tail. Filthy, battered, and limping slightly as she strode down the red carpet towards the dais with Twilight by her side, there was no mistaking her as... Cadence? A feeling of dread settled into Celestia's stomach as a loud gasp swept through the crowd and the feverish whispers increasing in intensity.

Beside her, Cadence – or was it? - gave an ugly sneer. “What? But how did you escape my bridesmaids?” she demanded, disdain ringing through her voice. One of them must clearly be a shapeshifter or magician some sort, but which? Please tell me that Twilight wasn't right all along? Yet she must be – the proof is standing here in front of me. Oh, Twilight!

Celestia's fears were confirmed by not-Cadence's next words. “Hmph. Clever. But you're still too late.”

Applejack's eyes were darting between the two identical princesses. “I-I don't understand. How can there be two of 'em?”

The Cadence on the floor announced loudly, “She's a changeling. She takes the form of somepony you love and gains power by feeding off your love for them.”

This time, not only did the crowd gasp, Celestia could see them starting to panic. Don't stampede, if one of you bolts it will be a massacre in here.

Not-Cadence's face was twisted in hate – and a second later she vanished in a column of green fire. Inside it, Celestia could see the familiar form of her beloved niece being burned away to reveal a blackened, chitinous form with the fangs and insectoid wings of a huge changeling. Celestia spread her wings and launched herself through the large open window behind the dais. Beyond the shimmering purple shield of Shining Armour's protection spell, thousands of similar, twisted black forms hovered, hammering away with magic and even their bodies. Celestia felt her mouth dry in horror; the reports had been true! Her head jerked around, thinking to raise the alarm, but she could already see guardsponies galloping into position around the ramparts of the castle. That's all very well – but there's a whole city at stake. Shining Armour's spell had better hold, or they will be everywhere!

Behind her, the massive Changeling continued to gloat but fortunately, the wedding guests seemed transfixed by her. Don't bolt, don't stampede, don't bolt, Celestia pleaded in her mind. The only thing for it was to destroy the queen, and then hope that Shining Armour could be revived enough to hold off the rest of the swarm. She circled around to the window again in time to hear the queen announce, “Soon, my changeling army will break through. First, we take Canterlot. And then, all of Equestria!”

Well, if that's not a straight line for an entrance, nothing is! Taking a deep breath, she plunged back into the throne room. “No. You won't. You may have made it impossible for Shining Armour to perform his spell, but now that you have so foolishly revealed your true self, I can protect my subjects from you!”

The changeling queen gave her a tight, feral smile, and settled onto the far side of the dais. Between them, Shining Armour continued to stare vaguely into the distance, his face slack and uncomprehending. Launching herself into the air to clear the befuddled stallion, she fired a lance of pure energy at the changeling, who intercepted it desperately with one of her own. Celestia's magic burned inexorably down on her opponent, beating back the green energy as the changeling squealed in terror.

And then, the insect opened one eye and smirked at her, and she felt the magic twist. Her own eyes flew wide as she realised the changeling's desperate ploy. Not a focussed energy lance, but instead a wild, concussive detonation that would take out half the throne room – and all the ponies in it! She found herself fighting not only to destroy the usurping bug, but also grappling desperately to wrap her magic around the green spell, to limit and contain and bind it to harmlessness. She had more than enough power, of course, but if she didn't measure its application perfectly she could make the changeling's destructive spell look like a foal's firecracker, and balancing the two conflicting needs took all of her concentration. The changeling was massively strong, stronger than she ever would have imagined, and the green beam was now beating back her own. In a flash she realised what had to happen. Either I win and kill everypony in this room and maybe even the castle, or I can take the hit and save them all. Closing her eyes, she let the changeling's spell close the remaining distance, and clamped her magic as tightly around it as she could, and hoped.

The hammer-like blow against her horn sent her spinning across the room, ears ringing and her head reeling sickeningly. Dimly, she became aware of Twilight cradling her head and calling her name. Twilight. Poor Twilight, how I have wronged you! But maybe you can keep this thing occupied – or even defeat it yourself? She forced herself to speak. “The Elements of Harmony. You must get to them, and use their power to defeat the queen.”

The six brave little ponies exchanged a look and bolted as one for the door, pursued by the changeling's mocking laughter. “You can run, but you can't hide!”

That was enough for the rest of the guests. Following Twilight and her friends' precipitous example, they, too, fled screaming for the doors. A few ponies were knocked over and trampled, but to her relief, the big changeling seemed happy enough to let them go. Even the stragglers and the poor, half-crushed victims of the stampede were allowed to limp away deeper into the castle and safety, even if only temporarily. Only Cadence and Shining Armour remained, and tears poured down Cadence's face as she desperately tried to snap her fiancé out of his trance.

A screech filled the air as Shining Armour's protective spell over the city finally failed, and from her position on the floor, Celestia could see the tiny black shapes darting down into the city. My city! My little ponies! Time, time, time, they need time – and so do I. Gah, my head...!

The slow clop of insectoid hooves told her of the changeling queen's approach. “So. Princess Celestia. You've grown fat on the love of your pathetic little subjects. Fat, and lazy, and weak! What a feast they will make for my changeling army! And I never would have dreamed that I might have you here, too, in my power. What better way to ensure our food source than to control their ruler? And you will make a perfect puppet for me to control!”

Desperately, Celestia tried to summon enough magic to squash the bug and damn the consequences, but the only result was a red-hot knife of agony down her horn and into her skull, and a brief flash of golden light which skipped off the queen's chitinous shoulder and detonated a second-floor alcove in a spray of gravel.

“Uh-uh-uh!”

The world reeled again as a hoof connected hard under her chin, and by the time her scrambled senses realigned she found her horn trapped against the floor under her conqueror’s hoof. The pain increased as the big changeling leaned forward and her weight bore down.

“I would break it off and use it to nail your porky flanks to the ground, but once I've broken your mind I'm going to want you to be able to use it. The power of a goddess, bent to my every whim!”

The changeling sneered down at her, pale-green venom dripping from her fangs and leaving smoking droplets on the floor in front of Celestia's muzzle, and despite herself, she flinched. The changeling’s triumphant smile widened as she leaned down. “Don't worry, it doesn't hurt. Much. Isn't that right, Shining Armour?”

She couldn't stop her eyes flicking to the dreamy-eyed figure of her Captain of the Guard, who nodded slowly. Without thought. Empty. Like a puppet. Time, time, time, I need more time! I can't allow this to happen to me! I can't allow this thing any sort of control over my power! She could destroy this world through sheer ignorance!

But there was no more time.

The fangs were dipping down towards her exposed neck.

There was no more time.

Her choice was made. With a silent prayer for forgiveness, she reached out and touched the sun.

The world disappeared in an eyeball-searing flash of white.

=====// \=====

She was floating, gently buoyed on the surface of some liquid. A choking wave of utter despair swept through her, and she forced herself to open her eyes and contemplate the horror of what she had done.

She was floating on a sea of molten rock, the air still thick with ultra-heated plasma gusting in raging streams away from the surface. Girdled and protected by her innate magic against the heat, she struggled agonisingly to her hooves, the scent of sulphur and ozone in her nostrils. The throne room was gone, as she expected. The castle was gone also. Slowly, she swept her eyes around the entire horizon but she was standing in the middle of a single, bobbing puddle of white-hot lava. All around it, smoke boiled into a rapidly-darkening sky, staining the brightness of her beloved sun's light.

The city on the rock was no more. Canterlot had been reduced to atoms – maybe less.

A rapid bubbling nearby threw Cadence to the surface, also protected by the immense power of her Alicorn magic. Trembling, the young mare staggered to her hooves and stared uncomprehendingly around her, her face a mask of grief and loss.

Celestia knew she was crying, but the intense heat flashed her tears to steam even before they reached her cheeks. “Cadence!”

Her niece didn't look at her. Instead, her eyes were fixed on a single spot in the hissing, spitting pool of lava.

Celestia knew what she was looking for. “Please, Cadence...”

Even as she spoke, she knew her words were futile. Shaking her head, Cadence's mouth slowly formed the word, 'No'. Then, lifting her muzzle to the sky and spreading her wings as she did so, Celestia felt the jolt as her niece severed her own magic. The pretty pink body instantly became a column of flame and seconds later in collapsed in ashes onto the surface of the liquid rock, where it was consumed.

Gone.

Gone like everypony else.

She didn't know how long she walked on that hellish surface, sobs tearing at her chest, but the surface had started to plasticise when the thunder of mighty wings heralded the arrival of a blue Alicorn. The touch of her hooves left little islands of solidified rock in her wake and for a long moment Luna simply stared at her, horror mingled with fear in her face.

Luna's voice trembled in terrible accusation, “Celestia? Celestia, what have you done?”

Between sobs, Celestia tried to explain, but instead of compassion or forgiveness or even understanding, her younger sister's face simply hardened more with every word. “I had no choice. I couldn't summon enough conventional magic to fight her off – and I couldn't possibly permit the chance for her to control my power. She would destroy this world on a whim!”

“So instead you decided to destroy it yourself,” Luna snapped. “You must have known what would happen.”

The accusation hit her like a blow. “I-I thought the throne room, maybe the castle, but this? No, I could never foresee this.”

“Would it have stopped you if it had? You couldn't bear to be humbled so you destroyed an entire city!”

That barb stung her to protest. “Luna, you know the power we wield! Should that fall into the wrong hooves-”

“They might obliterate an entire city? They might kill tens of thousands of our little ponies?” Luna flung a front hoof wide to encompass the smoking devastation. “Look around you, sister! Look on your work, Celestia, and despair! You have wrought your fears with your very hooves – yea, even tenfold, for now Equestria is utterly defenceless against the likes of Discord, or even Nightmare Moon!”

Celestia shied away from her sister's violent outburst, but forced herself to reply logically. Inside, she felt herself screaming, and her hooves shook with the urge to simply throw herself down and bawl. “The Elements will survive. Whatever they are may not be unmade by any force in this world – even us at the height of our powers. As for Discord, you know as well as I that when he bound himself to that misshapen body of his, he took on its frailties – even as we did. His statue cannot have survived this – and so neither can he.”

“You think the spirit of Chaos is so easily defeated?” Luna bristled again. “Discord as a physical being may be destroyed but the spirit that he embodied cannot! And now you remember the Elements? Have you grown so addled that you forgot whence you despatched Twilight and her friends? Do you really think that even under the changeling queen's control you could have stood against the power of the Elements? Especially when they would be allied to mine?”

“I-I don't understand,” Celestia stuttered. Her head reeled, and she needed time to think, somehow. Each new accusation was like a blow, and she was slowly sinking, unable to comprehend.

Oblivious to her inner turmoil, Luna raged on. “Where do you think I went when I realised what was happening? The very chamber of the Elements! Ready to help deploy their power either in your aid – or against you, as needs must! And now they are lost to us!”

Celestia repressed a low groan as she forced magic through her horn, laboriously crafting a summoning spell. Why were her thoughts so disjointed? Connections she knew she would have made without effort normally unravelled and flew away from her. Was it shock, as ponies called it? Perhaps that was it. “Not lost, Luna. Hidden for a time, perhaps, but not destroyed.”

If anything, the Princess of the Night's face darkened even further, but instead her wings lowered and she asked, “Where is Cadence? Surely she must have survived also?”

Celestia couldn't answer, but merely bowed her head.

Luna's temper faltered, and she murmured, “So it's true. I thought I felt it, but...”

“I couldn't stop her,” Celestia whispered.

“Does that surprise you? After you murdered her fiancé on their wedding day?” Luna spat wildly. “Damn you to the pits of Tartarus, you and your stubborn pride have ruined all of Equestria in an afternoon!”

“Luna, enough!” she cried, but her voice was plaintive rather than commanding, and her sister's contemptuous look was yet another dagger to her heart. I sound so... weak.

Far off to one side, six objects detached from the cooling lava and floated towards them. The precious metals of their settings had been lost but the distinctive gems themselves remained, embedded in clumsy fragments of cooling rock. “There, see?” said Celestia desperately, lunging forward to clutch them. “The Elements!”

Luna slapped them out of her hooves dismissively, “Trinkets! Baubles! Truly, this heat has addled your brain! You foal! They are worthless without their bearers, worthless!” she thundered. “They refused us a thousand years ago – do you really think they will subject themselves to you now?”

Celestia quailed from her sister's rage. “In time, new bearers shall arise,” she offered weakly.

“In time perhaps they shall. But what of Rainbow Dash and Rarity? And what of gentle Fluttershy, and Pinkie Pie? What of dear, honest Applejack and your beloved student, Twilight Sparkle?”

“Don't speak to me of them!” Celestia shrieked wildly. “Please, I can't bear-”

“Please? Nay!” Luna screamed back, “And they are but the tip of the iceberg, for nopony in Canterlot can possibly have survived! Do you begin to understand what you have wrought here? I speak of them because their lives are on your head, and the lives of countless others! I speak of them because to remain silent defiles their memory! If it galls your tender heart to be reminded of what you have wrought – I care not, for you deserve it!”

Celestia sank down onto her belly, weeping blindly, “Sister, forgive me! They cannot but please! I will do any penance, accept any judgement you care to decree!”

There was a long silence before Luna spoke, her voice as final as the slam of a tomb door. “Nay.”

Celestia looked up in shock and despair.

“I will not pass judgement upon you, Celestia, nor demand penance, for to do so implies that I have an authority which is not mine. Or that I have the wisdom to devise a fitting punishment that makes sufficient recompense for every life lost, for every family torn asunder, for everything you have destroyed. Instead, I will do something much worse; I leave you to live with the knowledge of what you have done. How you atone for your deeds is up to you – but only you will know when you have abased yourself enough. Let every pony in Equestria that you have wronged be your judge, but I will have none of it!”

With that, the midnight-blue Alicorn turned on her heels and walked away, taking flight a few paces later and arrowing straight up into the smoke-wreathed sky. Celestia raised a futile hoof after her, calling her name, but she didn't look back.

=====// \=====

Hours later, Celestia stood on the rim of the plateau that was all that remained of the once-great city of Canterlot. Rivulets of stone ran from the edge like candle wax, and on the plain far below roiling clouds of thick smoke rose in all directions, the relict of the wave of superheated air that had washed down from the city flattening and igniting everything combustible within miles. Here and there, her sharp eyesight could see the occasional Pegasus pony through the veils of smog, desperately trying to wrangle clouds into position to contain the fires, but their numbers were too few and the clouds far too sparse. Of Cloudsdale, she could see nothing, and could only guess at the toll that her power had taken on the floating city.

Closing her eyes, she reached out to that great and terrible vessel of light and coaxed it down below the horizon with the minimum of ceremony. She watched as shadows lengthened, but the light of the sun was not replaced by darkness but instead the infernal glow of a thousand fires. Turning away, she raised her gaze to the heavens, and froze. There, just cresting the horizon, was the familiar sight of Equestria's companion planetoid, the moon. Its familiar, white surface was stained blood-red by the drifting smoke, but that wasn't the only change.

In a throwback to a thousand years of solitude and anguish, the surface bore the darkened figure of a horned pony's head once more.

The Mare had returned to the Moon.

She was alone.

=====// F I N I S \=====

Author's Note

If you back a Princess into a corner, and she's desperate enough - what would be the result? Add some headcanon, throw in some dot-joining around the scenes shown in A Canterlot Wedding, shake well, and serve chilled.