After she had made her decision to help Blueblood, she immediately started working out a plan on how exactly she was going to do this. Having already failed to get Celestia and Luna involved, her only option left was to investigate his time spent at the archives and Library of Magic, which should be doable considering her status as element of harmony and personal student of Celestia. She could easily come up with a number of reasons to justify her visits if anyone asked, but she needed to be careful of Blueblood's own visits, as to avoid making him suspicious to her investigation.

It... turned out to be more difficult than she expected.

Requesting access to the archive logs wasn't a problem, as she expected, as pretending to want to help locating any missing volumes was more than enough to convince the head librarian to give her all the freedom she wanted. However, actually going through them proved to be much less fruitful than she hoped. Blueblood regularly visited, yes, but the logs don't actually show him loaning any books. Additionally, simply requesting to read certain works inside the archives are not recorded at all, provided you have the proper authentication, so she had no idea what he was actually reading, much to her growing frustration.

Striking up small talk with the archive personnel didn't teach her anything either. She disguised her intentions by mentioning she was interested in what other ponies came read from the archives, always eager to broaden her own horizons, but whenever Blueblood came up they all had but one thing to say. Every two days he comes here, records his entrance into the logs, and then finds himself a secluded spot to read a number of volumes, before putting them back and leaving as quietly as he came. Of course, the personnel were polite enough not to disturb him and thus had no idea what kind of volumes he was working with, so that was a dead end as well.

In summary, the archives frustratingly didn't turn up anything, so here's to hoping the Library of Magic would at least give her something.

And it did. But just not in the way she was expecting.

It was mostly the same story as with the archives, records showed that he did not take any books outside of the library, nor did he interact with the personnel much if at all. He stayed here far longer than in the archives on any given day, but that doesn't help Twilight if she doesn't know what he had been reading. However, there was one curious point multiple ponies mentioned.

Blueblood often seemed to vanish when no-one was looking.

Now, while the Library of Magic may be a vast place, rivalling even Canterlot Castle when it comes to sheer size, ponies don't just up and disappear in this place. And yet, there have been numerous instances when one of the librarians simply could not find Blueblood at all for one reason of the other, before giving up altogether and going on with their business. Even so, without fail, he is always seen exiting the library shortly before closing time, obviously proving that he did not, in fact, vanish.

So what was going on here?

Twilight was wandering around in the section he was usually seen in, 'ancient history', and wondered how one pony could disappear like that. This section was somewhat removed from the main body of the library, and as such she found it to be very quiet, not even a pony in sight. Actually, why was he even here? Did his problem involve history in any way, or...

Looking around, Twilight noticed that this section wasn't just quiet, but rather that it was exceedingly not-used, much to her personal dismay. A lot of the books seemed dusty on the outside, as if few ponies ever intentionally came specifically to this section to search for a book. A quick trip to one of the assistants confirmed that, yes, not many ponies these days seem to be interested in this world's history any more, fewer still in the oldest parts of it, and as such very few loaned books come from the ancient history section. Not that he minded, as no one should feel forced to learn about something that doesn't interest them, but Twilight thought it a shame nonetheless.

Shaking herself out of her thoughts, she focused on her mission again. Twilight started considering what she had learned. One pony disappears in a place very few ponies visit, and, thinking logically, there are only very few ways one could do that. Most obviously the answer could be teleportation, a breeze for Blueblood, but why come specifically out here to do so? No, that isn't it. Whatever it is Blueblood is searching for, he thinks the answer lies here.

Another option would be that Blueblood simply hides well enough out of sight as to not be disturbed, perhaps even using the aid of an invisibility spell? No, the assistants have been very clear on this, when he's gone he's gone, no ifs and buts about it. Additionally, any and all uses of magic are strictly forbidden inside the library unless authorised, even for a high-ranking pony like Blueblood, which meant immediate expulsion if discovered. Blueblood could not risk that.

So that leaves but one option remaining. A secret passageway.

Twilight's eyes widened. Could it be? Yes, it absolutely could. If her memory serves right, this library was not always a library, but was initially build as a central military command center back in the starfall wars, even before the construction of Ponyville. So if this center was ever under attack, the existence of several passageways attackers were not aware of would prove vital in evacuating the commanders that resided here so long ago. Surely some of these passages would still be undiscovered but operational, even if the current library staff was not aware of them?

Twilight thought for a moment, and her horn lit up after she made sure the coast was clear, powering a spell. Reasoning further, even if Blueblood did use such an unexpected route in such a deserted but unconventional place, what was the guarantee that no-one else would stumble on it as well? If Twilight was right, she should soon find... Yes! She can sense it, the magic detection spell worked! Right... there, on a bookcase filled with pre-Equestrian legends, tucked away from sight.

An ignore spell! And a mute spell as well?

How clever. Blueblood, or somepony else, the signature wasn't clear enough, cast an ignore spell near the top of the shelf. Ignore spells are supposed to do precisely what their name implies, make you uninterested in whatever they are cast on. While they are usually used in medical treatments of addiction or against break-ins and thefts, it surely can be used to make sure no-one accidentally discovers an ancient passageway.

The mute spell, on the other hand, seemed to encompass the entire bookcase, implying but one thing. This thing can move, and completely silently at that.

Twilight started to feel giddy. This was her first and perhaps major breakthrough. Behind this bookcase is a passage or room that holds the answers she is looking for. After making sure that no-one was in the immediate area, she cast a temporary anti-magic spell on herself to only counteract the ignore spell, and quickly moved to the bookshelf. It took but a few moments to realize two of the books were fake, and pulling them back caused the entire shelf to rotate backwards, without so much as causing a squeak, revealing a magically lit spiral stairway going down.

Twilight was at a loss for words, and did not move for several seconds, her head filling with more questions. What was going on? Why would Blueblood need to use such an elaborate contraption? Should she head in? Should she alert Celestia? Should sh-

Twilight was pulled out of her thoughts when she saw the bookshelf move back to its original position, presumably working on some kind of mechanical timer to continue its function of being an unassuming bookshelf with books no one wants to read. This snapped Twilight out of her trance, and she shook her head, determined to find out what lay beyond.

Pulling on the books and moving the bookcase once again, she stepped forward and went down the stairs.

Half a minute later, and feeling like she went five levels underground, she finally came upon a door. And behind that door was... well, a large room. But what was in that large room might as well have made it feel like the single most important place Twilight had ever entered.

The restricted section. Twilight was exhilarated.

She had heard the legends, of course, that within the Library of Magic existed a section that contained only the most ancient, awful and destructive of books. Entire walls lined with shelves full of old books, every single one capable of forever changing the face of Equestria if they were ever discovered, but she never once considered it might be actually real. She even asked Celestia herself about it once she learned of it, but she firmly denied the existance of such a place with an unwavering voice, quelling any doubts she might have had.

So was even Celestia not aware of this place, or did she not want Twilight to find out? Twilight was sure that the library staff at least did not know about this place, or they wouldn't be running around looking for Blueblood when all this time he had simply come down here. She also realized just why the entrance is so difficult to find, yet so easily accessible without anypony noticing once you actually do. For who would think to search for a room filled with potentially world-destroying knowledge amongst 'the boring books about history no one cares about anymore'?

But it exist! It's real, and it's filled with books about... Actually, what are they about? Twilight was so amazed by what she discovered she hadn't even thought to actually see what's in here!

With a small spring in her step, she decided to look around. She hadn't forgotten about her mission, of course, but surely one peek wouldn't hurt? Getting her bearings first, she surveyed the room. Except for the single large fireplace at the other end, a large mirror to the side, and a large table full of scattered books and scrolls in the middle, the room was divided into a lower and upper level, both lined with bookshelves, and those shelves lined with dusty, decaying books. Much to her dismay, these books were clearly not cared for, and time had taken its clear toll on them.

Levitating books at random while walking along the shelves, reading some of the titles she could actually make out before putting them back caused a shudder to run through her coat. Void magic, biological immortality, large scale geo-formation, erased history, species experimentation, even starfall itself... the rumours were true. Any one of these, if leaked out, could change the entirety of Equestria, perhaps even the world. Much to her surprise, she even found a few works seemingly written entirely in one of the bald apes' many dead languages. Just how old was this collection?

I should not be here was the overwhelming feeling she got after seeing one terrible work after the other, and her earlier feeling of awe was quickly replaced by one of dread.

Quietly shivering, Twilight shook her head and made her way to the large table, eager to find out what the hell Blueblood had been doing here and getting out as fast as she could. Once she arrived, looking at the books and scrolls haphazardly scattered about, she perhaps finally discovered what had haunted him for so long.

Alicorns.

It was all about Alicorns.

What?

Taking a moment to double check, she confirmed that, yes, every single book and scroll, albeit each in varying levels of disrepair, was about alicorns.

Twilight Sparkle was confused. Why would Blueblood go through all this trouble, all this secrecy, only to research something for which he has three live subjects to work with? Surely Celestia, Luna or even Cadence could help him? Twilight had a vague feeling that she understands the when now, why Blueblood only began to display his changed behavior after the return of Luna, obviously an alicorn, but as for the how, why, what or maybe even who, she was still in the dark.

Luckily enough, despite how scattered everything was, Blueblood seemed to keep various bookmarks in several books, greatly easing her remaining search, and Twilight went to work.

Levitating one work to her after the other, reading the pages the bookmarks were on, however, caused even Twilight Sparkle a head-ache. Nevermind that most of these were written well before even Equestria's founding and contained a vastly different vocabulary and word structure she was familiar with, the parts she did manage to understand without thorough research were just... random? No, unrelated would be the better word.

This book talks about the history of alicorns, that one of their relation to the other species. A third book theorizes about their abilities, while the fourth documents their impact on the world at large.

This scroll talks about the lifetime achievements of one Grand Waltz, an alicorn she never even heard about, while that one bears the entire Princess family tree, going over five thousand years back.

Twilight didn't get it. This was a close as she was going to get, and she didn't get it. There was no common theme here, no unified idea that might help her out, and Twilight was getting desperate. Granted, a lot of these books contained information she never even knew about, but try as she might she could not figure out why Blueblood was specifically researching these works, nor why he felt the need to research them alone.

In her desperation, Twilight nearly glazed over a page in one of the many books she had skimmed through by now, until she noticed something unusual. The book itself was not unusual as far as she could tell, a book dedicated to master-level magic relating to alicorns, written some time after Equestria's founding by none other than Clover the Clever herself. No, the unusual part was that Twilight saw wet marks on this particular page, recent ones at that. And the only pony that has been here recently, Twilight supposed, was Blueblood himself. Did he spill water on the pages? Twilight wondered, taking the time to read the words in more detail.

Wait, what is this? It's hard to make out, but, "for one ... defy mortality they... give up... life itself."?

If Twilight was reading this right, and she could scarcely believe she was... this page was not about magic only alicorns could perform. It was describing magic that could make one become an alicorn!

Her head started spinning. Could this be it? Was this why Blueblood had changed? Was this the reason behind everything?

Did he want to become an alicorn?!

Twilight stumbled back at this, not believing her own words. No, no, that can't be right. You can't become an alicorn through magic. That's impossible. It just it.

...

Is it?

Come to think of it, has Celestia ever told her about her past? Had she always been an alicorn? What about Luna, or Cadance? Could alicorns even be born? Twilight could certainly not recall right now, her stress shooting through the roof and impairing her thoughts, and she kept creeping further and further from that damnable table.

Twilight was starting to see stars. A tiny part in the back of her mind screamed at her to run away, to find Celestia and seek her aid, but it was squashed by another part that wanted to continue her mission. Perhaps this was not what Blueblood was looking for, as unrelated as everything else was. Perhaps she should put the whole anypony-can-apparantly-become-an-alicorn thing beside her for now, first try to figure out why Blueblood was intere-

She did not realise she had never really stopped walking backwards, and jumped high when she suddenly felt something hitting her back, afraid that somehow Blueblood had come back and caught her in her snooping act. Quickly swivelling around, mind working overtime to come up with a reasonable excuse, she closed her eyes and let out a breath she didn't know she was holding when the saw it was just the mirror, the one she noticed when she first entered the room.

Calm down Twilight. Don't let the stress get to you... yet.

Opening her eyes again, she tried to see what Blueblood would have seen if he was indeed standing there, but... she failed. Twilight was confused again. Slowly but surely, she started to realize that this was not a mirror at all. The room that Twilight saw in it was similar to the one she was in, yes, but it was not filled with bookshelves, nor did she actually see herself in it.

It's... more like a window instead. She thought, moving the maybe-mirror-maybe-window a little forward to see what was behind it, only to find a solid wall. So not a window, then?

In disbelief, she turned back again and reached one hoof out to touch its surface, wondering what trickery was going on here. It is not a painting, that she could say for certain, and didn't notice her hoof actually touching anything until she looked at it.

It was going through the mirror.

Twilight jumped back in shock, pulling her hoof back, but the mirror remained the same as ever. And then it dawned on Twilight, her eyes widening. A crystal mirror. Of course they exist as well. Ancient gateways lost with time, rumoured to allow instant travel over massive distances without the active need of a unicorn provided you have at least two mirrors. They greatly outclass teleportation magic in both reach, capacity and ease of use, and have been theorised to even allow access to other, 'parallel', universes altogether.

And of course such a thing would be in the restricted section as well.

A quick directional spell later, and Twilight determined that this particular mirror surprisingly enough doesn't go that far. In fact, it goes right to Canterlot Castle, of all places. Did Blueblood have another crystal mirror in his personal quarters, allowing easy access to this place from there?

No, of course not, she already established that he enters the forbidden section through the moving bookcase, and he wouldn't need to go through the trouble of visiting the library and leaving a record of that if he had such an easy access nopony knew about. So where does it go then?

Well, no time like the present, and Twilight figured she wasn't going to find anything else in this room that could help her with her mission. She certainly didn't want to go near that table again for now. Of course, she could have taken a better approach than simply taking a few steps back, starting a gallop and jumping head first into the crystal mirror, but the ever rising stress might have been responsible for her certainly not-rational behaviour.

She expected to feel some kind of sickness, or at least vertigo from the sudden displacement as with her normal teleportation spells, but discovered she didn't feel... well, anything. It was somewhat warmer here, sure, but that's it. One moment she was under the library. The next, she was in Canterlot Castle, almost as if she simply took a step forward. Amazed at such a marvellous piece of technology, she tried to orientate where she was, until she heard sounds coming from a door near the far end of this new-but-same room, and decided to investigate. She hoped that this was the end of it, exhausted from all the recent discoveries she made, and made her way over. As she slowly opened the door though, she suddenly stopped.

She could recognize the sounds as a voice now, one which she knew particularly well. And the kinds of noises she heard, well... she suddenly remembered what the maids had told her a while ago, about how a certain pony had a secret mate, and she realised where she was.

Twilight Sparkle was now in Luna's personal quarters, and she was currently having some fun with a pony or non-pony no-one has seen enter or exit her rooms.

Twilight Sparkle started to connect the dots. Why Blueblood started behaving so distantly to everyone around him, when he started behaving as such, his visits to the forbidden section, all the secrecy, everything. She felt like she was about to pass out.

Against her better judgement, she opened the door a little further, trying to look inside. She wanted to be wrong, she wanted to just turn back and enter the mirror again, but she couldn't. Her determination wouldn't let her.

So she continued opening the door, and what she saw confirmed it all.

Blueblood was currently railing Luna from behind on her grand bed.

Oh Celestia.

And the world grew dark before her eyes.

Author's Note

I hope I buried the reveal well enough. Of course, if you took the time to read the description and see what contest this was written for as well as the content warning, you probably guessed this was where it was going, but I hope it was enjoyable enough all the same.