Harmony
Celestia watched as her student ascended into the heavens, her ponies breaking out in song all around her.
Twilight had surpassed all expectations, solved the mystery of Starswirl's last spell, and fulfilled her destiny all in one fell swoop. Even the nobles, who were always so resistant to disruption of the status quo, welcomed her with open arms. She was sure that Harmony would reign over the land for generations to come.
It was absolutely perfect.
It was absolutely wrong.
There was a lord of darkness. Sombra was his name, and King was his title. He ruled over the Northern Wastes with an iron hoof, claiming the Crystal Empire as his territory and its ponies as his subjects. When he attempted to extend his reach into the nation of Equestria, though, he faced two unstoppable forces.
Celestia, Sol Invictus, the Unconquered Sun, was the first.
The second was Luna, Deimos Selene, the Nightmare Moon.
For years, war raged on Equestria's northern borders. Thousands of ponies, crystal and otherwise, fell in battle. Such was the way of war. But in the end, after one last assault on the heart of the Crystal Empire, led by the Sun and Moon themselves, Sombra finally fell, and was banished into the freezing wastes. His Empire was wiped from the landscape, and the soldiers of Equestria returned home, leaving one outpost in the area to prevent any uprising.
Over a millennium later, as Harmony spread across the land, when a guard finally announced the return of King Sombra and the appearance of a Crystal Empire, Celestia was extremely surprised. But she knew exactly what to do.
Nothing.
She instead sent her student, the illustrious Twilight Sparkle, and her small band of friends to face the dark lord. To anypony that asked, she said that this was to serve as a test... the greatest test of the young student's life.
It was meant to be an exercise in futility, a chance for Equestria to relearn the importance of standing strong in the face of adversity. But Twilight returned, victorious, mere days after she had left.
Sombra was not the warlord he once was. He was, quite literally, a shadow of himself, driven back and destroyed by a device called the Crystal Heart, powered by love and happiness of all things.
As the delighted young student pronked out of the throne room, all decorum forgotten, Celestia drove a hoof into the ground, leaving cracks in the impeccable marble surface.
"Behold, behold!" The populace cheered their new princess, even as its current ones watched from the Canterlot spires.
The Solar Princess shaped her face into a carefully crafted smile, the one that always fooled everypony around her. But then, there wasn't such a thing as deception, was there? In the magical land of Equestria, ruled by Harmony and presided over by its immortal princesses, there was no need for such a thing.
Celestia was no stranger to deception. Subterfuge and misinformation were part and partial to the political machine in the early days of Equestria.
Now, perhaps, not so much.
So when she had felt its undercurrents once again, Celestia was thrilled. It had come as a rude shock, obviously, when she learned that the fae queen, Chrysalis, had kidnapped and replaced her beloved niece, Cadenza, in an attempt to stage a coup.
Chrysalis has cut her, deeply. But Celestia had no choice.
She let Chrysalis continue to work her magic, digging deep into the roots of the political system. Anyone who could have seen through the ruse was conveniently shunted out of the way. And the stalwart Captain of the Guard was reduced to a simpering puppet, even as Celestia gave him the order to erect his shield over the entire city.
Twilight Sparkle, of course, attuned to the magic of Harmony around her as she was, noticed the lies immediately. But the deception ran so deeply and thoroughly that everyone else was convinced otherwise. Such simple minds her ponies had!
Celestia bore only the smallest amount of guilt over having to tell her faithful student that she was wrong.
In what can only be called coincidence, Twilight found the true Princess Cadance, returned to the surface world, and exposed Chrysalis for all that she was. Celestia rose, knowing what she had to do to restore normalcy to Equestria...
She had to lose.
She had to let Chrysalis win, even while she possessed the power to destroy her and turn her to ash. Chrysalis was sure to prevail now.
But Harmony, once again, eked out a victory. Twilight restored sense to her fallen brother, and together, he and his new wife drove the changeling threat away with a single stroke.
The day had been just perfect.
"Everything's changing, Celly."
Celestia flinched at the unorthodox nickname, but relaxed at the sight of a brown unicorn stallion with a short gray mane and tail, dressed in a smart blue sport jacket over a starched white dress shirt.
"I suppose it is," she replied.
"Can you handle it?" Upon Celestia's wordless insistence, he continued, wiggling an eyebrow at her. "Can you handle an eternity with Twilight Sparkle, the Princess of Know-It-All-dom?" The stallion put a hoof to his goatee, stroking it gently. "I think I'm going to run that one by her, see what she thinks of it."
"Twilight Sparkle has done well," Celestia replied, successfully telling the truth and completely failing to answer the question all at once.
"You know, she really has, hasn't she?" He ribbed her in the side, pointing down at the parade below. "I mean, look. Their magic of friendship turned mean old Mr. Discord into a nice guy! Who'd have thunk it?"
Celestia nodded, without smiling. "Who, indeed?"
Celestia was, perhaps, the only one left who remembered what Discord had truly been; he was a demon... a lord of chaos, who threatened to tear the universe asunder for the sake of entropy.
For Equestria's warrior princesses, the Unconquered Sun and the Nightmare Moon, it was an adequate challenge. Only after years of fighting did they manage to seal him away. But his corruption lingered, a slow-acting poison that incrementally chipped away at the bond between the Sisters until it failed entirely.
When he returned, Celestia knew the course of action she needed to take... and again, the Elements of Harmony were called into action. For a time, it looked as if they, too, would fall under the influence of Chaos.
But Harmony was absolute.
Discord was no demon, not anymore. He was little more than an imp, content to disrupt the lives of a single town before being swept away by the magic of Harmony.
Celestia could not let this stand.
On her order, he was freed once again. Perhaps, given a second opportunity...
But Harmony had already exacted its toll on the trickster god. Given a second opportunity, Harmony dulled him further, altered his very nature... and turned him into just another slave to its will.
Not even Chaos itself could undo what Harmony had done.
"Brava, Twilight!"
The noise returned Celestia to the present time. To her right, her sister stomped wildly, cheering Twilight on as she soared through the sky.
When Celestia cleared her throat, she flinched, looking back at her with embarrassment. "I apologize, sister. But truly, look at the fruits your labor has wrought!" Luna announced, pointing at the graceful flying lavender streak in the Canterlot sky. "All that you planned for her has come to pass! Aren't you happy for her?"
Celestia grimaced. "Of course I am. I couldn't be happier."
One thousand years of peace.
Three hundred sixty five thousand, two hundred and forty two days of tranquility.
Eight million, seven hundred sixty five thousand, eight hundred and eight hours without major incident.
Five hundred twenty five million, nine hundred forty eight thousand, four hundred and eighty minutes where, no matter what happened, Equestria prospered.
Celestia was glad that her sister was returning from the Moon. Now somepony could share her madness... or better yet, end it entirely.
Luna, all elegant black and radiating power, alighted on the balcony atop Canterlot's tallest spire. Celestia, in regal white, with a rainbow-hued mane billowing in the magical breeze, was already waiting for her.
"Sister... what have you done?"
"I... I am sorry, Luna..."
"Can't you see? Harmony... it's smothering the land. It's killing it."
"All I wanted was... I didn't want to fail again. Not after losing you... I wanted... I needed to keep Equestria safe, and peaceful, and perfect... just for you."
"This is not Equestria, sister. It's a pale imitation. It's a tale out of a picture book. It's a filly's fantasy world!"
"But..."
"The world needs conflict! It needs adversity in order to grow! Without it, it will atrophy, shrivel up and blow away in the next strong breeze."
Luna and Celestia shared a long, meaningful look.
"Equestria needs a villain." Luna heaved a great sigh. "It is fortuitous, then, that I should finally return."
Celestia looked with love on her younger sister, her equal in every way. "Can it be done? Can you reverse this curse?"
"I cannot be sure," the black alicorn said. With a flash of cerulean, her old armor appeared on her body. She looked every bit the warrior princess she was a millennium ago. "But I'll be damned if I don't try."
So it was that the Nightmare Moon took wing again, threatening to destroy the world as everyone knew it to be. And from her tower, hidden from the world, the Unconquered Sun prayed that she would succeed.
But... Harmony... prevailed... and the world refused to change.
"Life in Equestria shimmers, life in Equestria shines!"
Celestia was alone. Sombra, Chrysalis, Discord... even her own sister, reduced to a pale imitation of herself... nothing could wrench Equestria from the iron grip of the curse of Harmony.
Her wish was coming true.
"I know for absolute certain, that everything is certainly fine!"
Nothing bad would ever happen to Equestria, ever again.
"Everything... yes, everything... yes, everything is certainly fine! It's fine!"
And it was all her fault.
"Yes! Everything's going to be just fine!"