On a day a few decades ago, a group of mundane mortals mistakenly thought that they could categorize the concept of knowledge. That they could contain the nexus of worlds, that the Foundation could lock up the platonic concept of a Library.
It was a learning experience, at least.
On that day, a Doctor pored over notes and maps, looking out the window of the temporary base of operations that had been set up for the incursion.
Not an invasion, not an assault. An incursion. That's the phrasing the Doctor preferred. Ideally it'll be more or less a scouting mission, something to gauge the relative scale of the Library.
Three groups, coming from three separate points. Triangles were meaningful shapes, right? The researchers weren't sure if they were or not. They weren't entirely sure of any of this, as a matter of fact.
Of course, only the very top of the command chain knew that. Everyone else was told that there were measured risks involved. It wasn't exactly a lie. Just stretching the truth.
Everybody took their stations, three entrances in three doorways in three small towns. Each in contact with the others, each planned to the minute.

The Library was as it always is. I would say it was a day like any other, if there were proper days in the Library. Or, at least, across the entire Library. Sometimes night is necessary.
The Library bustled as they always did. Sweeping, grand shelves stretching for miles, stocked with the knowledge of eternity; some towered higher than the sky, others were barely a foot stool's height. Every so often one might stumble across a small clearing, filled with couches or tables or whatever else the Library's visitors brought from elsewhere.
Speaking of which, at any given time beings from across time and space were scattered across the stacks, each unique but also not. An unfortunate alchemist, a skittish goblin, a peculiar cat, a soulless shapeshifter, an actual alien, a literal deer, a blindfolded witch - so many different lives meeting in the nexus of knowledge.
Throughout the Library stood Doors, each unique in itself, and each a literal Door to innumerable Ways. Each Way had its own Knock, and each Knock was unique in itself. Nothing was repeated in the Library, except when it was.
There was one corner of the Library, like most others, a clearing with a few tables and desks whose origins spanned across hundreds of years. This was one of many sitting spaces, and normally wouldn't be of special note in the Library; that is, until the moment a certain Door opened.
There was a hard, fast Knock on a tall wooden door set between two shelves. It looked as if it could be set in a farmhouse or old church, dusty and worn and heavy, heavy wood. It creaked a bit before a heavy boot kicked it open, and three dozen black-clad Agents of Red team swarmed in, weapons raised. Seven hundred feet away, another door burst open much the same way with Blue team's entrance; and three hundred feet from that one, another door with Green team's entry.
A single shot was fired, and screams began to ring out.

A field commander shouted as the Door slammed shut. The Doctor called over the intercom, demanding somebody retry the Knock - in a moment the Door opened again, and it opened to a different expanse of shelves. Agents rushed in.
Unfortunately, this was a different Way, and this corner of the Library was Restricted. An unfamiliar human voice began to wail as the Door slammed shut again.

In moments the Library was in chaos. The shot was taken by a newer Agent - nobody's quite sure who exactly, it happened so quickly - who got spooked and fired at… something. The moment the shot struck a shelf, the Library exploded into motion.
There was a palpable shift in the air as the Library came to life. The group huddled close together, tranquilizers and proper shots flying in focused bursts. A small black cat vanished into thin air the moment she was noticed, and a woman that looked almost like a schoolteacher dove behind a chesterfield; a man in a long coat drew his own pistol before being gunned down by the Agents, his body falling limp but bloodless on the marble floors.
In the distance more screams rang out. Agents shouted to one another, realizing the Door behind them was gone. Someone authoritative called for them to ‘find the others and regroup' - and they started off into the maze of shelves.
Then the Librarians came.

The Agents were scattered, gunfire and dozens upon dozens of voices screaming and calling out among the stacks. Occasionally gunfire rang out, and was swiftly silenced.
One of the Red navigators was in a panic, desperately trying to reopen the shut Door. Several other agents huddled close, ready to lay down suppressing fire if anything else approached; he didn't care, he just needed the damned Way to open.
There was a click and it finally swung open; the others looked back as he fell through into a hall of words, his voice perfectly silenced as old grey light poured into the Library like fumes. After several moments, an unfamiliar arm reached back out, closing the Door behind him.

They repeated the Knock over and over, each time afterward opening to a different place. A nearly solid wall of flame, an impossibly cold desert, an abandoned apartment covered in fungus; everything seemed lost.
The Doctor called his superiors. They would be unhappy but unsurprised.

Researchers at each entry point struggled to stay on the same page, each wracking their brains, trying to figure out a way to salvage the day. The Doors refused to work properly, the Agents were divided, the commanders were silent, the phones were busy… it was a nightmare.
Back in the project HQ, the Doctor hung up the phone, expression grim and sick.

Almost a lifetime away, an Agent from Blue was in a panic, crouched between a bookcase and a ratty old armchair. She peeked out to see a tall, mouthless figure practically appear out of thin air among her colleagues. It swung a heavy brass lantern with an eerie, silent grace, taking another Agent's head clean off; it was unfazed by gunfire, silently neutralizing the threat posed to the Library, like a white blood cell would neutralize a germ.
The cowering Agent squeezed her eyes shut, the sounds of carnage and gunfire ringing in her ears for a further eight seconds… before suddenly stopping. Trembling, she opened her eyes - the Library before her was immaculate, the MTF and everything else they brought gone.
Standing several feet before her was the figure, mouthless, a lantern hanging from its hand. Or… rather, the lantern hung from one arm.
It silently stood before her, holding its hand out. The Agent took it and was gone.

The Door was shut tightly again. Agents and researchers shouted amongst themselves, trying to figure out what happened. Minutes passed before the Way burst open on its own, a small group of Agents stumbling out. They scrambled and shouted, confused, ragged, hurt; it seemed like they had been gone for days.
Before anybody could react, one screamed, dragged back into the door before it slammed one last time, the old, grimy handle falling off.

Elsewhere, Green group was completely fragmented. One splinter made the attempt to move out and try to find the other Doors. It started with about twenty agents. Then about sixteen. Eleven. Ten. Eight… Five. Three.
Then there was only one Agent left. He began to run, the stacks growing taller, the air growing heavier, the light dimmer. It began to feel almost haunted, but not; it was the kind of feeling that you get when you know a place was once beloved, but has been silent for far too long.
He turned a corner, finding himself among towering shelves that stretched taller than anything, with books filled with forgotten stories.
Lost, he saw a towering figure atop a shelf, a regal beast with a man's face, looking down with bored curiosity. The beast smiled when the Agent laid eyes upon it, eyes filled with thirst and delight.
The Agent turned to run and ran headlong into a tall, crowned creature with a smile carved into its face. He began to weep, collapsing against the books, terrified and doomed. Somebody who couldn't be recognized said words of comfort to the lost Agent, who realized he had made a mistake.
The other strange figures watched as a Librarian climbed down, watching politely as the Agent tried to gather himself. In a moment the crowned thing scooped the Agent up like a baby, and leaned up, handing him off to the Librarian.

A pair of Agents ran, brothers chased by shapes and movement they didn't dare look at directly. They turned a corner, the first Agent bowling over a frightened child with iridescent eyes, the other jumping over; to where, they never really considered. They just needed to escape.
They ran for a long time, longer than they realized. Eventually they realized that the books were all gone… and the world around them had changed.
Light played above them, and they looked up to see a vast thing, like a titanic pillar on squat legs, a cage filled with innumerable lanterns planted atop its head. Branches like antlers strung with more lights stretched from the cage, twinkling like a tree filled with stars. The living thing's many arms reached throughout the hall, the scraping of quills on paper filling the brothers' ears.
It looked down at them, and they fled.

In the end, nobody knew what happened.
Like in everything else, every single person who was part of the incursion had a different story. Some seemed similar until one read into the subtleties, others had entirely unique experiences that no one else shared. Most contradicted the others.
Recordings of the day only helped muddy the waters further. Some showed chaotic images and sounds of alien worlds, others involved people who had not even been on the trip - that is, if they went on the op at all. Or had even been born yet.
Some of those involved had vivid recollections for the rest of their lives. Some remembered nothing but entering and exiting the Doors. It was like time skipped them over when they went through the Way.
The most peculiar thing happened thirteen days later. A group stumbled through Blue team's door, dressed in the tatters of their equipment, their hair greying and their bodies scarred and decrepit. They claimed to have been trapped in the Library for decades, and only just were able to leave. They each told fantastic tales of their own, of giants who stoked fires, of fantastic lost countries, of black dogs, of breathing ideas, of things they had no words to describe.
Many still remain. Lost souls who broke the rules, they must be punished for their transgressions. And they will, just as countless have been punished before them, and countless more will after.
It is a learning experience.