A/N: this is a what-if based on the events of Chapter 88 of the marvelous fic Harry Potter and the Methods of Rationality, so the rights belong to JK Rowling and LessWrong, respectively.
There are SPOILERS for that chapter, if you haven't guessed.
…okay, shoot me, I had to write this.
When Harry's Patronus left Hermione's side, she was already informed of the situation.
She was already running from a very big, very ugly mountain troll down a deserted corridor.
Already winded, but not yet panicking.
Because there was help coming, and she only had to keep ahead of the troll – why, she could fly away if she so wished.
She snorted, because really, how do you get Hermione Granger on a broom if not by sending a monster after her.
and they had to talk about his Patronus sometime soon
But the broom failed her.
And everything else, which she could spare breathe to take out of her bag, failed her, too.
And Hogwarts was so unbelievably, ridiculously empty.
And bright daylight poured in through high, narrow windows on one side.
It was then that fear burst in her.
faster
Because back in September, playing at being a Scientist, Hermione wondered why the third most perfect killing machine in all of Nature could be frozen by something as mundane as a ray of a totally non-magical star. (No, Mr. Potter, I don't have proof!)
As to why the ray shouldn't have been reflected by the Moon or any other planet before hitting the target, well, that was another question.
And she had been proud of actually thinking of that.
need air
slow down
no time
Why does a non-magical something influence a magical something? And not in Potions making, which seldom used living things and never – sapient ones? (To her knowledge). She resolved to answer that one when she graduated.
never
Okay, speculated then Professor Granger, maybe trolls hadn't been so vulnerable from the start, but a powerful Mage cursed their whole breed – not a smart one, but certainly powerful; though maybe that was the best he could do.
correction
maybe he was Bad and designed trolls for night-time attacks
maybe he was really Bad and that was the best he could do
and someone made improvements
just for her?
or were all trolls now sun-proof?
stumble
wall
skinned knees
go
She had meant to discuss it with Harry. There was little info in the library. Harry was busy. There was homework!
And she had told herself that magic didn't make sense.
Hermione felt her legs folding beneath her, threw out an arm not to slam into another wall and almost broke her fingers; her eyes watered from pain.
And the predator behind her grew nearer.
but not yet just behind
'What..'
She raised her head as if out of water – sweat running down her burning face – and saw a picture of a wizard surrounded by geese as tall as himself.
'Help,' Hermione wheezed out. The wizard blinked myopically. She pushed away with a gasp, and soon – too soon – heard the club crash into the stone.
So the picture was smashed.
And she was still running.
why?
This was a mountain troll, and it hadn't trampled her into the floor in seconds?
Hope flared briefly, but she was a General and it was clear, it was clear that –
it was herding her
something more terrible was waiting out there
Hermione hurled her bag over her shoulder.
think
open air?
no
where?
here
She dove sideways, into a narrow cul-de-sac, through blinding stripes of powerless sunlight, skidded, fell into an alcove. Mistake. Her movements were restricted, and there was zero protection from –
The troll turned the corner.
Idly lifted its weapon.
Advanced a step, crossing a harmless band of light from a high, narrow window. It was like some special effect in a Muggle movie.
where was Harry?
'Incendio!'
Missed.
'INCENDIO!'
Missed. It moved so quickly!
'Glisseo!'
Wasn't even noticed.
'Somnium!'
What?
Well it does have a brain!
Another step. It was toying with her.
think
She couldn't cast the Killing Curse.
This thing was impervious to the usual means.
Nobody was coming to help.
Harry didn't know the details.
She couldn't flee.
think
Another step.
Why did the Sun work? Why didn't the Sun work now?
Let it be solvable. Please. Let the troll be enchanted.
'FINITE INCANTATEM!'
It held out a hand, tenderly cupping the bright beam in its palm.
Her hands were cold. She was… crying?
think!
Harry made weapons from everything. When he wanted a sword, he made a sword.
Useless – the troll maintained constant Transfiguration on itself. It could only be defeated in one piece.
So? Apply energy, like Harry did that time with photosynthesis.
Useless – no source of energy, and nothing could penetrate its armour.
because stone didn't
conduct light
in most cases
Feeling nothing much, she raised her wand and twitched it once, twice, thrice and four times, and her raw fingers moved as if of their own will.
Harry believed she could do it.
There was no reason why she couldn't do it. Aside from hiccuping.
The troll leaned forward.
Mom, I will be home for summer
She whispered the words.
And won.