Princess Celestia wasted no time in explaining.

"Twilight, a long time ago... a little over a thousand years, I was visited  by a most peculiar pony. Or, I should say, I found him peculiar at the time. This pony gave me explicit instructions to speak with a pony named Twilight Sparkle regarding these matters once something called the "Mirror Pool" was mentioned by Twilight Sparkle herself."

Twilight was stunned. "M...me? But... how is that possible? How can a pony from so many years ago know about me?"

"This ponies name was Starswirl the Bearded. Perhaps that answers your question?"

"You mean... THE Starswirl the Bearded? The most renowned conjurer in all of Equestria?"

"The one and only. You may not realize it, Twilight, but you will achieve great things. Starswirl always had a strange habit of studying history in both directions," Princess Celestia chuckled at this, as if reliving some distant memory.  "You can imagine my surprise the day I dropped in on your magic exam when you were a filly. The name "Twilight Sparkle" was one I had nearly forgotten. And yet it isn't until now, so many years from then, that I can finally fulfill Starswirls instructions."

"And what might those instructions be, Princess?"

Princess Celestia began her slow trek back to the window, before suddenly turning and using her magic to close the door to her office. After she had done so, she spoke. "As I have explained, the Mirror Pool is not merely a pool with magical properties, but a gateway. And now, in another realm, that much is known. As a matter of fact, it is known through quite a large number of alternate Equestrias."

"Meaning what, exactly? Those Pinkies may try to return?"

"Oh, heavens no. If that were Starswirls great fear, then I would regard him as much of a madman now as I so foolishly did then. No, Twilight. What if another one of those realms were not, shall we say, as...diplomatic as ours. What if, say, in another Equestria, Princess Luna and I had been unsuccessful in our efforts to thwart Discord?"

"Then... he'd have easy access into our world!" Twilight realized, cursing herself for not considering this sooner.

"Exactly. However, whether or not Discord is the problem remains to be seen. You see, Starswirl was significantly vague in his futuristic predictions. I must apologize for this, but much like yourself, I badgered him for more information, but received none. I'm sure Starswirl had good reasons for holding it back from me, but at the moment it is his secrecy which has prevented me from knowing for sure what lies beyond the mirror pool."

Twilight thought about why Starswirl would have been so secretive about this. What good reason would he have to withhold such important information, from the Princess no less.

"Is it safe to assume that we should take the Elements of Harmony with us?" Twilight asked.

"I'm sorry, I truly am, but your friends cannot accompany you on this journey. You need to do this alone. Which leads me to my last instruction. No matter what the consequences, no matter how important you may think it is, you must promise not to interfere with the lives of the other ponies," Princess Celestia paused, contemplating exactly how she should word her next point without it sounding too terrible. Deciding it was impossible, she continued. "This means that if you are the only pony standing in the way between the life or death of another, even your friends, you must..."

"Let...them die? But... how? How can I do that?"

"Twilight, your interference in another realm brings both of ours in grave danger. You need to let things occur in the natural progression of time. Whatever does not directly correlate with the safety of our world does not concern us, and I need you to promise you will allow these events to occur."

"I... I promise, Princess," Twilight sighed, but then thought about how unlikely a life and death scenario would be in Ponyville in which the bearers of the elements of Harmony could not handle it alone.

"Good," The Princess replied, before turning and lifting a scroll of her desk. "Now then, lastly, this scroll has been in my possession ever since that visit from Starswirl. Please take it with you, and use it when its necessary, and only then."

Twilight nodded, and the two shared their farewells. As Twilight turned to leave the room, Princess Celestia spoke.

"Ah, I forgot to mention, do you still have your Nightmare Night costume in your possession?"

Twilight had been unaware that Celestia had known about it. "Yes, Princess..."

"Good. I think it would be wise to take it with you."

The sun was high above the trees as Twilight walked through the Everfree Forest, her saddle-pack filled with what Princess Celestia had instructed her to bring.

Ahead, was a thick forest of brambles, knotted together in a nearly impenetrable wall of foliage. Twilight used her magic to separate them, and saw that the path twisted downwards towards an underground cave. Light was emanating from something near the end, and Twilight concluded that this had to be the spot.

Deeper and deeper through the cave she traveled, when she saw what she had come searching for. The pool was glowing with light like the sun, and Twilight remembered what the Princess had instructed her to do.

The tip of Twilight's horn began to glow, dim at first, as she tried to remember exactly how the spell Princess had taught her went. When she became confident she remembered, she closed her eyes, the tip of her horn glowing brighter and brighter, before she heard a large splash. Opening her eyes, she saw the crystal clear water quickly loosing its reflective qualities and growing dimmer and dimmer, the supernatural light fading away. Suddenly, there was an explosion of light, forcing Twilight to once again close her eyes, this time not with concentration but with an instinctive reaction to not blinding herself. When it was all over, Twilight opened her eyes cautiously and examined the pool. The previously beautiful pool was now of a dreary and mysterious hue, a silver fog hiding herself. Before she could change her mind about it, Twilight leaped into the murky water.

There were no bizarre lights or sparks of blinding colors as Twilight traveled between the two worlds. She was simply thrown out of the pool and back onto the spot where she had jumped in. Twilight looked around cautiously, trying to determine if it had in fact worked. Outside, the noon-day sun was gone and the light of the moon could be seen through the hole in which she had emerged. Twilight stopped to make sure she was undamaged by the endeavor, before trotting out into the night.

The brambles she had separated had returned, but much thicker. Twilight could see that moving them out of the way could be difficult. Instead, she focused on the clearing peaking out from the forest of thistles. In a quick flash, she had teleported into the clearing, and back into the forest.

The Everfree Forest was significantly different. It seemed more well-kept, as if it were frequently inhabited by ponies for a long time. There was a clear, defined path to Ponyville not far from where Twilight was. Twilight turned to look at the mirror pool cavern and found it was nearly impossible to spot through the brambles. If she hadn't emerged from the other side, she would never have known it was even there. Twilight realized that she was probably the first pony in this Equestria to ever have seen the mirror pool.

Walking down the path, Twilight couldn't help but be amazed by the millions of stars dotting the night sky. She was surprised that the mirror pool not only altered space, but time as well.  

After a significant time on the path, Twilight could see the lights of Ponyville ahead.

Not long after Twilight was out of the forest did things begin looking strange in Ponyville. Twilight looked for a long time at Fluttershy's cottage. The cottage was in a state of disrepair, it had obviously been abandoned for perhaps decades. Its design was significantly different, too. Fluttershy had taken extreme measures throughout her life in Ponyville to ensure than the cottage was just as much of a home to her animal friends as her own, but none of that was evident here. The cottage was a tiny, ugly ramshackle.

The clock tower visible from Fluttershy's cottage was also gone, in its place a large industrial smoke stack spewing black fumes into the night sky. More smokestacks were visible lining the outskirts of Ponyville, Twilight could see their own trails of smog pouring into the troposphere. There was very little vegetation, the few trees straggly and looking as delicate as paper. From the looks of them, they were long dead, only preserved by the lack of anything around to interfere with them. The only trees Twilight saw flourishing were native to the Everfree Forest, and looked out of place in the familiar landscape of the Ponyville Outskirts. Typically the sunlight decimated these intimidating looking plants.

Twilight approached Fluttershy's cottage, gazing through the rickety door supported by only one hinge.

"Fluttershy?" Twilight called, hopefully, knowing full-well she was not around. What could have happened to her?

Twilight returned to the path and began trotting towards Ponyville, at a quicker pace than before. As she approached Ponyville, she was shocked to see a huge chain-link fence encasing the town. A light humming sound told her it was electric. The path continued on to a large gate, guarded on both sides by royal Pegasus guards. Their physical appearance was virtually the same as what she remembered, but their armor was a dark violet. They saw Twilight traveling down the path from a distance. As Twilight approached, she saw one of them whisper to the other, and the other turn and stare directly at her. At this moment, a surge of fear shot through Twilight Sparkle. As the two guards regarded her with their blank, unforgiving stares, Twilight decided the best course of action would be to turn around and run back into the forest, knowing full well these pegasi would be able to out-fly her with ease. Just as Twilight was about to begin her escape, the two guard ponies held their hoof to their foreheads in a salute, before opening the gate for her to enter. Twilight breathed a sigh of relief and saluted back at the guards.

Twilight pondered the actions of the guards as she walked towards Ponyville. Apparently she was regarded as some sort of figure of importance here. Surely not a Princess, but the actions of the guards led her to believe that this worlds Twilight Sparkle was also a student of Princess Celestia.

Ponyville was, simply put, a ghost town. The poorly constructed buildings all had their shutters closed, and the small amount of ponies Twilight could see in the distance scurried from building to building in a nervous trot. Twilight passed a group of fillies who evidently had not noticed her.

"Good evening!" Twilight said kindly.

To Twilight's shock, the young ponies turned around in terror, before bowing down to Twilight. One filly spoke nervously. "G-good evening... Mrs. Sparkle!"

This greatly distressed Twilight. These innocent fillies were reacting to her with complete and utter terror. The filly that spoke was quickly shushed by her peers. Twilight walked past them, feeling guilty for some malicious deed she had not performed.

The town of Ponyville looked surprisingly similar to her recollection of it, but in a greater state of disrepair. All the ponies she passed bowed in her presence, in the same nervous fashion as the fillies. Protruding from the town was an immense tree, its few leaves a dark yellow. Twilight realized this was her library, in the same straggly condition as all the other vegetation in Ponyville. It, like the town she was now in, was surrounded by a large electric fence, this time unguarded. There were no lights on in the library, but it was obviously not abandoned. "Looks like Twilight Sparkle isn't home," Twilight thought to herself.

In the center of the town, where Twilight remembered a fountain being, was an odd structure.  Twilight thought it looked rather similar to something she had seen on the cover of one of those books Rainbow Dash was always reading to young fillies and colts on Nightmare Night. Its purpose or name escaped her at the moment.

Across from the odd structure was another building that Twilight was also unfamiliar with. Unlike most of the other buildings in Ponyville, this one was rather well kept. Twilight decided it would be better to get a close look inside the building, and started crossing the town towards it. It was as she was doing this that something disturbed her. She couldn't quite put her hoof on it, but something seemed...off about these ponies. Like...something was missing.

And then she realized what it was. No pony she had seen bore a cutie mark to identify their destinies. Even more distressing was the fact that other than the two royal guards, there had not been a single Pegasus or unicorn.