Dear Princess Celestia,

Today, my friend Pinkie Pie discovered something rather extraordinary. What do you know about the mirror pool?

Twilight Sparkle frowned at the frankness of her letter. It wasn't typical for her to send a letter asking a simple question, but she needed answers. After what she did, she needed closure that it was the right thing. She'd tried to rationalize that it could wait until morning, but every second that ticked by, she couldn't help thinking about where she had sent those Pinkie clones. She didn't kill them, she knew that much, unicorn magic is never that powerful, unless fueled by vicious hatred. It was a teleportation spell that she'd used, but using a teleportation spell without a destination in mind can sometimes have disastrous results.

Twilight paced around the room frantically, stopping every once and a while to check the clock as its hands crawled painfully slowly across the face. Outside, the sky was dark with the promise of morning in a few hours, but now, not even a star grazed the empty sky to make up the breathtaking celestial she loved so dearly.

"Relax, Twilight..." Spike murmured from his bed in the corner of the room, watching Twilight gaze out her window in the direction of Canterlot. "The Princess almost never sends mail this late at night..."

"You don't understand!" Twilight exclaimed, turning to face Spike. "This is important! Surely the Princess will understand!"

Spike started to object, when suddenly, his sentence was cut short by a loud belch.

The scroll fluttered to the ground, but Twilight dove across the room to catch it. Spike rolled his eyes while Twilight frantically unfurled the letter. The letter was as frank as Twilight's, but coming from the Princess, the urgency of the letter sent Twilight into a swirl of fear.

Your presence is urgently requested in Canterlot. Please come alone, Twilight. It is time.

Twilight watched the lonely fields pass from the windows as the train tore through the countryside. Outside, the first glimpses of sunlight broke the jet-black night. Twilight thought of the Princess, standing on her platform, the top of her horn exploding into a shimmering spectacle of light, scattering across the land towards the East.

The train broke into a tunnel, and the light disappeared. The train rocked gently, but this was the only sound that interrupted Twilight's own train of thought. At this hour, almost all the carriages were deserted. When the train finally tore from the tunnel, Canterlot could be seen in the crepuscular morning light.

Twilight tore through the castle courtyard and up the spiral steps to the Princess's office, where she would undoubtedly be waiting. Why had she been so frank and shocked with her reply? Did she somehow offend her?

Princess Celestia was looking out the window when she entered.

"Good morning Twilight. Thank you for coming on such short notice," Princess Celestia said simply, without turning her head.

"Princess Celestia! What's wrong? Has something happened?"

Celestia chuckled softly, before turning to face her student. "I suppose I should be asking you that same question."

Twilight hurriedly explained the events of the previous day. How Pinkie had cloned herself, but oh, please don't blame her, she didn't know any better...

"And so I teleported all the Pinkies back where they came from..." Twilight finished, exhaling deeply as the Pinkie Pie-esque rant was over. "You do know about the Mirror Pool, right Princess?"

"Oh, yes, all to well. I was wondering when this day would come. I supposed part of me was hoping it never would... but such hopes are illogical."

Twilight blinked in confusion. "Pardon me, Princess?"

"Twilight, the Mirror Pool is unlike anything I have ever seen. Pinkie Pie used it for cloning, but its also capable of a much greater purpose. Are you familiar with the multi-universe theory?" Celestia was now crossing the large room towards Twilight, wearing a solemn expression.

"That there is more than one universe connected to Equestria? A parallel version of our own?"

"Not exactly parallel, and yet not drastically alien, either. Different choices may be made by different ponies, and the world may vary greatly from what we know, but it will be Equestria all the same. Twilight, the mirror pool is a gateway to another world, perhaps an unlimited amount of worlds. Pinkie wasn't cloning herself when she found the mirror pool, she was extracting alternate versions of herself from these separate realities."

Twilight was lost for words for a moment, simply nodding her head to show she understood. The idea of meeting herself, without the burdens of time travel (she simply did not want to go through THAT again) was more than fascinating.

"Twilight, the time has come, for you to make your own choice, perhaps the most important of all. Through the mirror pool lies another Equestria, one with a history darker than ours, yet intertwined with ours. I need to ask you to do something that I would never entrust with any other pony in Equestria," Celestia was now face to face with Twilight, her solemn expression replaced with something Twilight had never seen of Princess Celestia. The Princess of the Sun was afraid. "You need to be the one to do this Twilight..."

"What... what about Princess Luna? Or Shining Armor? I mean, its not like I'm a figure of royalty or anything..."

Celestia chuckled softly before shaking her head. "If they could go, believe me, I would send them along with you, but unfortunately that cannot be so."

Twilight was annoyed by the Princesses secrecy, but knew better than to show it. "Is it...dangerous?"

"Not to you, Twilight. You are the most trustworthy pony I can send."

"Why though? With all respect Princess, I need to know all the details before I agree to..."

The Princesses solemn expression returned, causing Twilight to trail off. "I understand your skepticism, Twilight, but I don't know fully the reason myself. But I have seen the evidence before me. A long time ago."

Twilight Sparkle thought for a long time about those last words. She'd never known the princess to be oblivious to something, especially something so "dark" and "dangerous." But she knew one thing for certain. The Princess was relying on Twilight, and she was not about to let her down.

"I'll do it," Twilight said simply, and Celestia nodded.