"Turns out, Pinkie had always been a changeling, she just hit her head when she was young and forgot about it," Twilight's image explained to Celestia, her voice magically transfering from Ponyville to Canterlot thanks to a couple of magically enchanted mirrors.
"That does explain a lot, actually," the white alicorn chuckled.
"Anyway, I have to go now, I'll make sure to send you a copy of the spell once I've perfected it." The mirror's surface wavered as the flow of magic was cut and the princess was greeted back by her own reflection.
A few days later Celestia could be seen happily walking down an hallway in the castle, an opened letter held to her side by her magic. She was heading towards one of the larger rooms in the palace, where she had requested the presence of the majority of the royal guard. As she neared the place, her eyes were caught by a strand of ethereal blue mane rounding a corner.
"Luna," she called, "up already?"
The dark blue alicorn poked her head from around the corner. "Hello, sister."
"Do you want to join me for something fun?"
"What would that be?"
"You'll see."
The two walked together to the entrance of the hall, then stopped in front of the doors. A single guard was waiting for them, his armor different from the usual one.
"Are you still adamant about your decision, princess?" he asked.
"Of course."
"Of course. But I still feel the need to reiterate that checking the guards one at a time would be a much safer method."
"And also a lot more boring."
"Princess, this is a serious matter. It could result in a case of mass hysteria. Think of all the chaos that could happen if we actually find somepony."
"I was under the impression that you were trying to dissuade me. Now if you'll excuse us," she said, then walked up to a staircase at the side of the entrance, leaving the guard to once again wonder how Equestria had managed to exist for over a thousand years with her in command.
Celestia and Luna reached the top of the staircase and walked onto a balcony running along the top of the tall room's perimeter, overlooking the rectangular guard-filled space below. The white alicorn gave a look to the letter she was holding, then with a bright flash of light cast a spell that filled the whole room before dissipating. She looked attentively at the ponies down below, but much to her disappointment the only effect the spell produced were a couple of confused looks.
"So... what's supposed to happen?" came a voice from the alicorn's left. She slowly craned her neck to the side until she came face to face with Chrysalis, the changeling queen standing where Luna had been just moments ago.
"Sister? Something wrong?" asked the bug-like creature.
A flash of light later, they both were standing inside Celestia's room.
"Explain," said the princess in a dry tone.
"What do you mean, sister?" Chrysalis asked back.
Celestia simply looked at her. The changeling seemed rather confused, then lowered her eyes to see her chitin-covered body.
"Oh."
The pair stared at each other silently for a couple of seconds.
The changeling queen's body started to slowly change, deep blue fur covering her legs as her wings were replaced by feathered ones.
"You know I'm not blind, right?" hissed out the princess.
Chrysalis quickly reverted her transformation.
Another silent pause.
"Chrysalis?"
"Yes?"
"Where is my sister and what are you doing here?"
The queen gave out a low sigh. "I should have known this day would come. Oh what a tragedy this is, that my secret has been revealed at last. If only there was a way to undo this terrible fate. But alas, the cruel tides of destiny have decided to strike down upon the life I had for so long struggled to maintain. Nevermore shall my soul be lifted from this shadow. Woe is me, Chrysalis, queen of all changelings, for I have fallen prey to my own scheme."
"What in my name are you talking about? And what has any of this to do with Luna?"
"Poor Celestia, such a heavy burden I have to confess. In truth, your beloved Luna- No, I can't bring myself to tell- But, but you deserve to know. It's my fault, and I have to face the consequences. Princess Luna,-" Chrysalys paused dramatically, "-never was!"
Celestia's head tilted slightly to the side. "Excuse me. WHAT?"
"It was I, Chrysalis, disguised as your sister, ever since you were only a little foal."
The princess cleared her throat. "Excuse me." A glass of water flew in from the nearby bathroom, enveloped in her golden magical aura. She drank its content and then returned it to its original position. "WHAT?"
"It's the truth. I originally planned to take control of Equestria by gaining your trust, but the love you felt for me! I could not bring myself to break your heart, revealing you had never had a real sister."
"That makes no sense."
"How delusional you are. Tell me, do you perchance remember the day Luna was born? Of course not, Celestia. I appeared one day, and your little brain accepted me as a part of your life. It was so obvious too. Did you really not notice how your sister came back only once I was gone that wedding night here in Canterlot? How silent the other changeling I had instructed to replace me was while I impersonated Cadenza when all three of us were together? Or when I ran away after the destruction of my throne? How it was from Luna's room that I came when I captured you, and no sounds nor alarms were heard?"
"But Nightmare Moon-"
"A clever use of my changeling powers and a few light tricks. It was such a perfect motivation too, the role of princess of the night really was giving me little love to feast on. But you surely can't have been so blind as to not see how not a single changeling attack was made during the thousand years I spent on the moon. Not once did you meet Chrysalis in all those years, despite how common our battles had been before. Battles all fought in the day might I add, when you believed your younger sister to be sleeping."
The alicorn had to admit, everything she had been told sounded plausible. There had been no confrontations between her and Chrysalis in the time of her sister's absence, and she was really young when Luna had supposedly been born. And she had never seen Chrysalis in the same room as a talking Luna. Still, she did not believe the changeling.
"If you really want to prove to me that what you say is the truth I demand you tell me something that only the real Luna would know." She patiently watched for any signs of indecision on the queen's face, but was only met with a sad look.
Chrysalis leaned forward. "On your eighteenth birthday-"
A shiver run down Celestia's spine. No, this couldn't possibly be happening. There was no way she could possibly know.
"-you ate the whole cake on your own before the party," the changeling whispered to the alicorn's ear.
Celestia's mouth was left hanging open. "I- You- W- What... B-bu-but..."
From the position she was in, the queen could almost hear the sound of a tocsin going off inside the other's head. Cogs grinded and flew off inside her brain, until finally the chaos inside her mind came to a screeching halt.
"ALL THIS TIME?"
"Yes, sister. Although I suppose I should stop calling you that."
"BUT WHY?"
"Because I loved you. You became the bigger sister I never knew I wanted, and it would have broken my heart to break yours with the truth. Farewell Celestia," Chrysalis said before changing back into the form of Luna and leaving the room, the alicorn sitting there, contemplating the way her reality had crumbled with a wide-eyed stare.
From the door on the side of the room opposite to that the changeling had walked through the tired and yawning figure of princess Luna entered the room.
"What was all that screaming, sist-" she stopped, swearing she'd just heard the sound of an explosion followed by a number of screws ricocheting off of a metallic surface.
"Luna?"
"Yes?"
"Did you tell anypony about that thing that happened on my eighteenth birthday?"
"You mean the cake one? You said it was a secret between alicorns, so I only spoke about it to Cadence. It was back when she was getting marri..."
"..."
"Oops."
The sound of glass shattering echoed from the depths of Celestia's brain.
Then the whole city of Canterlot, the whole mountain even, shook from the thundering roar coming from Celestia's room.
"CHRYSALIS!"
Just outside the castle, a particularly fast moving guard could be seen accelerating in his run.
