Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

Right now Mr. Filch's brown clothing was disarrayed and soaked with sweat, his shoulders were visibly heaving as he breathed, and his everpresent cat was missing.

"Troll -" gasped Mr. Filch. "In the dungeons -"

Minerva McGonagall stood up from the Head Table so quickly that her chair fell to the ground behind her.

" Argus! " she cried. "What happened to you?"

Argus Filch staggered forward from the huge doors, his upper body streaked and dotted with small crimson dots as though someone had spattered steak sauce over his face. "Troll - grey - twice as tall as me - it - it -" Argus Filch covered his face with his hands. "It ate Mrs. Norris - ate her all up, in just one bite -"

Minerva felt a stab of dismay in her other self, she hadn't liked the other cat very much but the two of them had still been felines.

An uproar started from the Great Hall. Severus stood up from the Head Table, somehow doing so without drawing any visible attention to himself, and strode out the huge doors without another word.

Of course, Minerva thought, the third-floor corridor - this could be a distraction -

She mentally consigned all such matters to Severus's care, drew her wand, raised it high, and let out five sharp cracks of purple fire.

There was stunned silence but for Argus's broken sobs.

"It seems we have a dangerous creature loose in Hogwarts," she said to the faculty at the Head Table. "I will ask you all to aid in searching the halls." Then she turned to the stunned and watching students, and raised her voice. "Prefects - lead your houses back to the dormitories immediately!"

Percy Weasley leaped up from the Gryffindor table. "Follow me!" he said in a high voice. "Stick together, first-years! No, not you -" but by that time the other prefects were raising their own voices as a renewed babble sprang up.

Then a clear, cool voice spoke under the sudden rush of sound.

"Deputy Headmistress."

She turned.

The Defense Professor was calmly wiping off his hands on a napkin as he stood up from the Head Table. "With respect," said the man of unknown identity, "you are not expert in battle tactics, madam. In this situation, it would be wiser to -"

"I do apologize, Professor," said Professor McGonagall, as she turned toward the great doors. Filius and Pomona had already risen to follow her, with Rubeus Hagrid towering over all of them as the half-giant stood up. She'd been through similar experiences too many times, at this point. "Sad experience has taught me that on occasions such as these, it is not a good time to take any advice the current Defense Professor may offer. Indeed, I think it wise that the two of us search for the troll together, so that no suspicions may be cast upon you for any untoward events which occur during that time."

Without any hesitation, the Defense Professor swung smoothly on the Gryffindor table and clapped his hands with a sound like a floor cracking through.

"Michelle Morgan of House Gryffindor, second in command of Pinnini's Army," the Defense Professor said calmly into the resulting quiet. "Please advise your Head of House."

Michelle Morgan climbed up onto her bench and spoke, the tiny witch sounding far more confident than Minerva remembered her being at the start of the year. "Students walking through the hallways would be spread out and impossible to defend. All students are to remain in the Great Hall and form a cluster in the center... not surrounded by tables, a troll would jump right over tables... with the perimeter defended by seventh-year students. From the armies only, no matter how good they are at duelling, so they don't get in each other's lines of fire." Michelle hesitated. "I'm sorry, Mr. Hagrid, but - it wouldn't be safe for you, you should stay behind with the students. And Professor Trelawney shouldn't confront a troll on her own either," Michelle sounded much less apologetic about this part, "but if she's paired with Professor Quirrell the two of them together can form an additional trusted and effective battle unit. That concludes my analysis, Professor."

"Adequate, for being put on the spot," the Defense Professor said. "Twenty Quirrell points to you. But you neglect the still simpler point that home does not mean safe, and a troll is strong enough to rip a portrait door off its hinges -"

"Enough," Minerva snapped. "Thank you, Miss Morgan." She looked to the watching tables. "Students, you will do as she said." Turned back to the Head Table. "Professor Trelawney, you will accompany the Defense Professor -"

"Ah," Sybill said falteringly. Beneath her overdone makeup and mess of shawls, the woman looked rather pale. "I'm afraid - I'm not entirely well today - indeed, I feel rather faint -"

"You won't have to fight the troll," Minerva said sharply, her patience taxed as usual when dealing with the woman. "Just stay with the Defense Professor and do not let him out of your sight for an instant, you must be able to testify afterward that you were with him at all times . " She turned to Rubeus. "Rubeus, I am leaving you in charge here. Keep them safe." The huge man straightened at this, losing his glum look and nodding proudly to her.

Then Minerva looked at the students, and raised her voice. "It should go entirely without saying that anyone leaving the Great Hall for any reason, will be expelled. No excuses will be accepted. Am I understood?"

The Weasley twins, with whom she'd been making direct eye contact, nodded respectfully.

She turned without another word and marched off toward the hall doors with the other Professors behind her.

On the far side of the room, unnoticed on the wall, a clock showed 12:14pm.

...and he still didn't realize.

Tick.

As Harry stared with narrowed eyes at where the Professors had gone out, wondering what was actually going on and what it meant, as the students came together into a more defensible mass and wands flicked to levitate the tables out of their way, Harry still didn't realize.

Tick.

"Shouldn't the Professors all have formed up into pairs?" said an older Gryffindor student whose name Harry didn't know. "I mean - it'd be slower, but it'd be safer, I think -"

Tick.

Someone else replied to this, raising her voice, but Harry didn't catch much of it, the gist was that mountain trolls were highly magic-resistant and incredibly strong and could regenerate but they were still noisy so if you heard them coming, it shouldn't be that hard for a Hogwarts Professor to wrap them up in Vadim's Unbreakable something something.

Tick.

And Harry still didn't realize.

Tick.

The crowd noises were subdued, people were talking in low voices to each other while they glanced around, listening for the sound of a crashing door or an angry roar.

Tick.

Some students were speculating in whispers about what the Defense Professor could possibly be trying to achieve by smuggling in a troll, and whether he was angry that Professor McGonagall had caught on to his attempted distraction, and what it was a distraction from.

Tick.

And the thought still didn't come to Harry, not until after all the students had formed a mass of perhaps a hundred bodies patrolled by proudly grim-looking seventh-year-students with their wands all pointed outward, and somebody suggested doing a headcount, and someone else replied sarcastically that this might have made sense on some other day, but right now practically everyone was gone for the spring holiday and nobody really knew how many students were supposed to be in the room, let alone if any were missing.

Tick.

That was when Harry wondered where Hermione was.