Derpy was roused the next morning before sunrise by a gentle rapping on her door. "Wake up, Derpy, dear! You don't want to miss the Celebration!"
This was followed by a hearty BANG BANG BANG of a hoof on the wall. "Hey Derpy! Hey Dinky! Rise and shiiiinnnne, party tiiiimmme!" Pinkie yelled.
Dinky rubbed her eyes sleepily and sat up in her bed.
Derpy smiled across at her from her own bed.
"Ready for the Celebration, Dink?"
"Uh huh!"
They washed up, split a muffin between them in lieu of breakfast, and quickly trotted down to the square and into the main rotunda of Ponyville's Town Hall. They'd left it a bit late, and had to squeeze in at the back of the crowd. Derpy lifted Dinky up onto her shoulders so her daughter could see. Looking over the heads of the crowd, they could see Twilight and the other ponies standing at the front of the hall, near the dais.
A bold trumpet fanfare split the air, accompanied by a chorus of songbirds conducted by Fluttershy. A spotlight swept down to the dais, landing on a tawny-flanked, grey-maned pony: Ponyville's administrative head, Mayor Mare.
"Fillies and gentle-colts!" she called as the music came to an end. "As Mayor of Ponyville, it is my great pleasure to announce the beginning of the Summer Sun Celebration!"
A cheer rose up, accompanied by much hoof-stamping.
"In just a few moments our town will witness the magic of the Sun rise, and celebrate this, the longest day of the year!"
Derpy happened to glance upward, toward an upper window of the Hall through which the light of the Moon was falling.
She could see the face of the Moon itself framed in the window. And as she watched, she saw the Shepherd Stars suddenly close in, and appear to slip behind the Moon.
Derpy blinked. Okay, so I wasn't seeing things last night.
And then, all at once, the Mare's shadow itself flashed and faded away, leaving the Moon's face as bare as a filly's flank.
Derpy's mouth dropped open. The Mare! Where is she!
"And now," the Mayor continued, "it is my great honor to introduce to you the ruler of our land; the very pony who gives us the Sun and the Moon each and every day; the good, the wise, the bringer of Harmony to all of Equestria... Princess Celestia!"
The music rose and so did the spotlight, to the curtained balcony.
Rarity was standing ready beside the curtain ropes and she gave them a tug, drawing back the curtains with a grand flourish, perfectly on cue.
Except for one thing. The balcony was empty.
Princess Celestia wasn't there.
There was a long, shocked silence.
"Uh, remain calm, everypony!" the Mayor called, in a We're Officially Not Worried tone of voice. "There must be a reasonable explanation!"
From the balcony, Rarity called out, "She's GONE!"
A horrified gasp went up from the assembled crowd. A dark, shadowy cloud was rising from the balcony where Celestia should have appeared. It was like a translucent gap in the air itself, through which the nighttime sky was visible. It swirled and rose in the air, flinging off dark streamers of itself. And then in the center of it all, appeared a mare.
Tall, night-black, with a crescent moon mark on her flank, wearing night-steel armor and helmet, she spread black wings and flourished a fierce-looking horn.
Derpy's eyes went wide. Alone out of the crowd she smiled in amazed joy. The Mare in the Moon! She heard my wishes to her! She's here!
"Ahhhh, our beloved subjects!" the Mare's booming voice easily filled the entire Hall. "It's been so long since we've seen thy precious, little, sun-loving faces!"
From the crowd Rainbow Dash bounded into the air. "What did you do with our Princess?" And then she tried to launch herself at the dark pony. She was only stopped by Applejack, who had grabbed Rainbow's tail in her teeth.
The black mare just laughed. "Why? Are we not royal enough for thee? Dost thou not know who we are?"
The Mare's wings beat powerfully, lifting her from the balcony. She suddenly darted close to Fluttershy, scaring away the bird chorus and causing Fluttershy to cringe in midair, scared out of her wits.
"Dost our crown no longer count? Now that we have been imprisoned for a thousand years?"
She swept over toward Rarity. An edge of the shadowy cloud had formed itself into a sharp point, forcing Rarity's chin upward.
"Did thee not recall the legend? Did thee not see the sign?"
"I did!"
The Mare turned at that, and looked down to the floor of the Hall. Twilight stood forward, facing her.
"And I know who you are. You're the Mare in the Moon -- NIGHTMARE MOON!"
At this, the crowd of ponies gasped. Even those who didn't recall the legend of Nightmare Moon could recognize a Seriously Bad Thing when they heard it named.
All except Derpy, who was still struggling to work it all out.
This is the Mare, she thought. The Mare in the Moon! I wish to her every night! Why is she like this?
"Well, well, well!" Nightmare Moon chuckled. "A pony who remembers us! Then thee also knowst why we are here!"
"You're here to --" Twilight felt her courage slipping away. "To --" She found herself unable to go on.
To grant our wishes? Derpy shouted deperately to herself. But she was becoming less and less certain.
"Mommy, I'm scared!" Dinky whispered.
"Me too, Dink!"
Nightmare Moon chuckled at the crowd's terror. She swept back to the balcony, and stood facing her terrified audience.
"Remember this Day well, little ponies! For it was thy last! From this moment forward...
The Night
Will Last
Forever!
And she bellowed a sinister laugh that shook the Hall, as the shadows swirled about her darker and darker.
Finally, Mayor Mare pulled herself together. "Seize her! Only she knows where the Princess is!"
Three of the Royal Guard Pegasi launched themselves into the air, aiming for Nightmare Moon. She merely glared at them. Then her eyes shone with power, and lightning bolts cut the air, sending the guards flying.
At the same time, the swirling shadows swept around her, hiding her from view.
And then the shadows leapt from the balcony, swept through the air across the hall, and flowed out through the main doors, leaving panic and chaos behind in the hall.
"Hold tight to Mommy, Dink!" Derpy turned and bolted for the exit herself, just in time to avoid the crush that formed as everyone else struggled to leave the building.
She raced across the square outside, coming to a halt at its far edge. In the distance overhead she could see the flickering shadows disappearing, speeding away above the tangled boughs of the Everfree Forest.
"Come back here!" called a voice from above her. Derpy looked up and saw Rainbow Dash, wings flapping, hanging in the air and staring after the disappearing shadows in frustration.
Behind her, she heard hooves pounding. It was Twilight, with Spike on her back, rapidly disappearing up the road towards the Golden Oak Library. After watching for a moment, Rainbow spun in the air and dove after her.
"What's happened, Mommy?" Dinky asked.
"The Mare in the Moon," Derpy whispered. "She's here. She's real! But she's angry. She's real angry --" All at once, she got it. "-- because no one remembers her! Except for Twilight," she added. "And for me."
"Oh! Derpy, my dear! And Dinky!" It was Mrs. Cake, trotting up to her in quite a state. "Well, I never expected anything like this! What are we going to do?"
Right behind her was Mr. Cake, a tall, lantern-jawed yellow pony. "We're going back to the store, that's what! To barricade ourselves inside." He nodded to Derpy and Dinky. "You're welcome to come with us, if you want!"
For a long moment, Derpy stared off toward the Everfree Forest.
Then she swung her snout around, collected Dinky, set her on the ground, and gave her a serious look.
"Go with Mrs. Cake, Dink! Mommy has to work now."
Dinky looked about to object. Then she nodded sadly. "'Kay, Mommy!"
"Come along, my dear!" Mrs. Cake said to Dinky, gathering her in. "And you do be careful, Derpy!"
"I will! Thanks, Mrs. Cake!"
She watched them leave, glad she had Mrs. Cake as a friend.
Then she turned back to her plan.
First, she was going to need a way to hide herself. A disguise, I need a disguise. She immediately galloped away down the street leading to the Costume shop. She didn't like taking advantage of its unlocked door, but she reasoned she could pay Ms. Harlequin back later -- assuming there was a later.
And then she spotted the cardboard box sitting on the porch. It was the one she'd delivered the previous day. Its contents had been unpacked, and the box had been left out for collection. Just a big empty box.
Derpy's concept of disguises was somewhat literal-minded.
Perfect, she thought. Nopony will recognize me in that!
She nabbed it by a flap and dragged it back up the street towards the post office.
- - - - - - - - - - - - - - -
Nightmare Moon, in her shadow form, flitted over the dark and twisted Everfree Forest, covering the distance in minutes. She arrived at the ancient ruin at its heart: the Castle of the Two Sisters. And here she sought out the one thing that could possibly stop her.
Yes, the Elements of Harmony were here. No one had come to claim them. And no one ever would!
Satisfied, she indulged herself by materializing in the old throne room, with its twin thrones.
"Thou thought to imprison us? US? The Princess of the Night? I hope thee enjoys thy imprisonment in the Sun, dear Sister! For we will never allow it to rise, never allow thee any chance of escape! NEVER!"
And there were none who would challenge her! None who would dare stand against her! The ponies she had seen today were weak! Defenseless! Powerless! None of them knew of the legend! Except...
She considered. There was that one in the hall, who had called her by name. That one was a potential threat. Well, she would not be so for long. And then -- and THEN! Nightmare Moon would rule as the Night itself! Eternal!
With a mighty laugh, she exploded into her shadow form and rose above the Castle, coming about and flitting away across the Everfree, back toward Ponyville and a certain lavender pony who knew far more than was good for her.