The sun had just cracked over the horizon of Equestria's borders when the train carrying the six ponies and Spike had arrived back in Ponyville. Having to ride on the train all night was incredibly tiring for the ponies and Spike, with not one of them having been able to stay awake for the trip back home—which was fortunate for the others that Twilight had also fallen asleep, so they would not have to listen to any more of her dreams of grandeur while trying to get some shuteye. Upon the train's arrival to Ponyville's train station, the conductor woke everyone up so they would all know they've arrived at their destination. Everyone proceeded to exit the train; except for Twilight, whom was still set on delivering her sample to Princess Celestia. After bidding their fond farewells, the train departed from Ponyville and set off for the Canterlot, with Twilight growing more and more eager the closer they got to the capital city of Equestria.
After arriving in Canterlot, Twilight made haste up through the corridors of the castle towards Princess Celestia's throne room, where the princess was busy managing the many stacks of paperwork dealing with issues in Equestria. After knocking on the door and being allowed to enter by Celestia, Twilight came forth into the throne room, much to the surprise of the princess. Upon greeting Twilight, Celestia dismissed her assistant, informing her that she would take care of the rest of the paperwork later, and walked over to greet Twilight. Twilight greeted Celestia in return, also bowing before her. Then, with the formalities out of the way, the ardent Twilight was at last able to present that which she had brought all this way.
Using her magic, Twilight opened her saddlebag and pulled out the cloth bag, moving it in front of Celestia to properly show the princess her discovery.
"Princess Celestia," she began. "I have come here to inform you of a unique discovery I made whilst my friends and I were exploring the Equestrian sentry posts in the San Palomino Desert."
"Oh? And is what you have here that very discovery?"
"Yes it is, princess. Behold: my discovery!" she exclaimed proudly as she lifted the green matter out of the pouch and presented it to the princess.
"...my, how...interesting..."
"I know, right?? I found this bizarre substance on top of one of the tower bridges, and my friends also found additional quantities scattered amongst the debris of the other sentry posts. I'm not totally sure what it is yet, but before you reply, let me tell you: this is absolutely not mold."
"Well, if that isn't what it is, then what do you believe it to be?"
"Like I've said, I'm still not sure exactly what this element is, but ever since I discovered it, I have formulated several likely facts as to the physical properties and history of this mysterious matter."
"Really? Well, I'm eager to hear what you've found out."
"He he, okay," Twilight responded, a smidge giddy. "Well, first: considering the placement of this and the other samples me and my friends had located, and the fact that the sentry posts have not been used in ages, it is my belief that this substance has been in that location since the posts had been in use. Second, I noticed how flimsy and yet still brittle the surface of this matter was upon my first contact with its surface. Conversely, the inner body beneath the frail shell was a bit more plush, and did not shatter when I forced my hoof upon it. I surmise that back when this first appeared on the bridge, the entire form was as soft as the inside is now, and over time, the outside hardened into a tougher crust, which then grew weaker after centuries of exposure to the elements."
"Yes, that is quite interesting, Twilight."
"I know, right??" she responded excitedly, but then quickly reverted to her more professional tone. "Yes; thank you, Princess. Additional information discerned by me is that, whatever this substance may be, I am absolutely certain that it did not appear there naturally."
"Really? Then how do you think it did appear at the sentry posts?"
"That's the question, your highness—one of many, in fact: What is this strange matter? Where did it come from? And why did it show up only at the sentry posts?" Twilight placed herself deep in thought—she greatly desired to unravel the unknown history behind the mystery substance. But how could she go about unearthing its secrets? She already had placed several questions pertaining to this enigma—but now she needed to ask herself the right question to get herself going on the right track. Such a task would be laborious for most anypony—but Twilight was not just anypony—she's shown her keen abilities to decipher riddles and provide further understanding of previous unknowns in the past, and it was these abilities that would grant her the power to do so once more.
Twilight paced back and forth inside the throne room, with full focus having been placed on her rumination. "What could it be?" she asked herself. "Where did it come from? I know I can think of a means to further my investigation," Twilight thought about all the facts again, attempting to discern something new that she had not taken notice of previously.
"Let's see...hmm...if...this substance...was found only around the sentry posts...then that means....that something had to have happened long ago at the sentry posts while they were still in use! Yes! I'm sure of it!" she exclaimed with exceeding optimism. "...but, none of the history books I've ever read about Equestria ever mentioned any kind of incident occurring at the sentry posts. Hmm..." what had just seemed like a breakthrough had once again become a quandary for our young unicorn, which forced her to begin her thought process all over again.
"Let's see...since this happened so long ago...princess, do you know anything about the history of the sentry posts that might uncover some of the mystery behind the peculiar material?"
"Me?" Princess Celestia responded rather quickly. "Why, of course not, Twilight. Don't be so...silly..." the princess stuttered as her eyes darted back and forth across the room.
"Princess, why are your eyes moving back and forth like that?" Twilight asked, bemused by the princess' odd behavior.
"Well...umm...” Celestia started to say, but halted, unable to speak any further. Beads of sweat formed on Celestia's visage, with her pupils having grown smaller in combination with the feelings of nervousness and fear rising within her.
Twilight was growing more and more bewildered by the strange actions of her mentor. Normally, she was perfectly collected and never so tense. She kept up the appearance and disposition of a ruler, but now those qualities seemed to be dissolving away, leaving only a mousy shell of her former self. Such a shift in character could not go ignored by Twilight for long.
"Princess, I can tell something is on your mind. You never act like this normally. ...is there something you want to tell me?"
Celestia sighed. "I'm afraid I can no longer keep this secret from you, Twilight."
"Secret??" Twilight responded in astonishment. "What secret??"
"Come. It will be easier to explain if I show you as well," Celestia left the throne room and out into the halls, with Twilight following after. The princess led Twilight to her bedroom, shutting the door behind them. She then asked Twilight not to mention what she was about to be shown to anypony else—and that included Spike. Twilight agreed, getting more rapt with whatever it was the princess was going to show her.
Princess Celestia walked over to her fireplace, and used her magic to snuff out the fire that had been burning there. She then moved closer to the opening of the fireplace and stuck her head inside, feeling a bit of the emitting heat from the still smoldering wood beneath her. She then placed her horn against one of the bricks in the fireplace's back wall. Magic was generated from her horn and into the brick, with the brick glowing vibrantly afterward. Once Celestia had taken her horn away, the magical energy went from the body of the brick into the spaces around it. Quickly, the magic trailed through the spaces between the rest of the bricks, until that whole section of the wall was lit up, leaving Twilight stunned and eagerly awaiting the outcome of this rare occurrence. In an instant, the glowing had vanished, and all was quiet for a few moments. Then, a small rumbling was heard, and the wall separated in two along the spaces between the bricks, and both parts of the wall receded into the sides of the fireplace.
Twilight was still speechless in the wake of this event—though on the inside she was quite giddy to have been a witness to a secret of the princess, and also that she was worthy enough to have been allowed in on the secret as well.
"Come," said Celestia, and she walked through the secret entrance. Twilight followed, unsure of what lie beyond the dusky corridor.
This tunnel was not only lengthy, but also much more winding than Twilight expected. Though the passageway became more lit by the small torches lined up along the walls of the tunnel, it still proved difficult for Twilight to discern the shape and position of the passage. However, while her sight was impaired, she could still feel the direction in which she was moving. Sometimes she went right, sometimes she went left, and at one point she felt herself going straight up in a spiral staircase—though that would have been obvious for anypony to figure out. Suddenly, Twilight sensed that the brightness visible in her eyes had increased. She soon detected that the light was coming from ahead of her and Celestia—most likely indicating that they were nearing the destination of whatever it may be that Celestia was going to present the young unicorn. The closer the two ponies advanced towards the luminescence, the more eager Twilight was becoming. The brightness increased more and more, until both ponies were engulfed by the luminosity. Twilight attempted to adjust her eyes to the bright light, and after blinking several times, she was finally able to see clearly. Twilight looked upwards, and was instantly taken aback at what she beheld.
All around her, a wide array of books and scrolls were laid upon multiple shelves. It was as if she had stepped into a one-room library; the shelves reached the top of the ceiling! Though the room was not too great a height, but still reached an impressive size that was capable of containing such a vast collection of knowledge. Even behind Twilight by the doorway there were numerous shelves holding countless numbers of scrolls and tomes. Many were even scattered upon the floor, due to the lack of space for accommodating such an immense throng of information.
Twilight's immensely widened eyes displayed her unadulterated absorption into what she saw as her own personal paradise. She ambled into the center of the room, spinning herself around as she gazed upon the pillars of literary wonder that surrounded the euphoric mare. In the depths of Twilight's fantasizing mind, she imagined all that she could read and learn from the scrolls and books in the chamber. She was so full of happiness and joy that she never wanted to be brought out of it. Unfortunately, this blissful epoch was suddenly interrupted by Celestia, causing Twilight's rapturous daydream to melt away, bringing her back to the less than perfect realm of reality. Twilight turned towards Celestia, and suddenly felt intensely embarrassed for having made such a display in front of the princess. She then regained her more dignified status and readied herself for that which the princess had to say.
"Now, Twilight, you may be wondering why it is I have brought you here."
"Oh, yes I am, princess, although I assumed it has to do with my question about the sentry posts. Does it?"
Celestia nodded. "You see, long ago, though not long after Equestria was first founded, there came a catastrophic event that caused vast ruin to Equestria and its inhabitants. It was a great time of tragedy for all, and was indeed the darkest time in Equestrian history," Celestia showed a deeply anguished expression, which was furthered by the single tear trickling down her face. Twilight was more surprised than sad at this point, but could see exactly how bad Celestia felt about this.
"After that incident," she continued. "The leaders of Equestria deemed it best to hide all knowledge of what had happened from the future generations of Equestrians, so they would never have to know that their country had taken part in this great tragedy, and every proper ruler or rulers of Equestria since then were able to keep the secret—but now I see that you are fit to know the secret for yourself."
"Really??" Twilight said, getting ecstatic again. "I really feel honored, princess, but; what exactly is it that happened in the past, and why was it so horrible."
"That is what I am here to tell you," Celestia then used her magic to pull over the rostrum in the center of the room. She then moved over to one of the second level bookshelves and began scanning the documents to locate what it was she needed. "Twilight," she said as Twilight had just seated herself before the rostrum. "You may wonder why it is this even troubles me so, even though it is apparent that I was not around back in that time. Well, I have always cared deeply for this land and all the ponies who reside within it, and just the thought of this having happened to even a single pony of this country...it just...just..." Celestia looked as if she was about to cry. Her tears increased in flow, and she was summoning all the strength she had to keep herself under control. Soon, the tears went away, and Celestia was back to her normal self. It was not that she was afraid of crying, but she wanted to tell Twilight this story, and that was a task that could not be done unless all her emotions were at more controlled levels. Though she did not lose herself in her tears, she had shown enough for Twilight to be more than concerned about her mentor. Whatever it was that had happened, it must have been gravely horrendous to have caused such pain in Celestia, and it was because of that display that now Twilight truly had all her focus on Celestia's historical narrative.
Finally, Celestia found what she had been looking for: a very large book located in the back of the shelf she was at. She placed this sizable tome on the top of the rostrum and opened it up to the first page. Twilight leaned forward and listened intently as Princess Celestia began reading to Twilight the record of the "War of the Equestrian First Reign".