"Ooh, yes, that," says the girl, pulling a fancily carved stick out of her pocket. "And for a demonstration--wingardium leviosa!" She gestures with the stick, and the antique-looking tome lying open beside her lifts gently into the air. She guides it up to shoulder height, then grabs it and tucks it under her arm. 

"If 'being the Heir of Slytherin' is a conjurable parameter and the circle doesn't have a binding in it, which it probably doesn't, then maybe? Who's Slytherin?"

"Salazar Slytherin was one of the four Founders of Hogwarts. Someone claiming to be his Heir is petrifying muggleborns, possibly with magic he left hidden somewhere."

"If they might only be claiming to be his heir then it's less likely I can accurately conjure them, I might get an actual descendant who innocently operates a tea shop or something while their identity is used by another party. Uh, can you describe in detail more about the petrifying, thing..."

"It's been two students, a cat, and a ghost so far. They turn up unconscious, all stiff, like statues. Except the ghost, he turned grey and they had to blow him around with a fan. The teachers say there's a potion that might cure them, but it'll take all year to brew. And . . . right before the first attack, I heard a voice nearby, talking about how it wanted to kill, but I didn't see what was talking."

"Name one of the students for me? Or the cat, I suppose."

"Creevey. Colin Creevey."

Little diorama of Colin Creevy at the time of his petrifaction?

Kid holding a camera up to his face. Patch of floor and walls. Giant menacing snake head way bigger than the kid!

"...you said he was petrified, not, uh, vanished without a trace into this here very obvious-looking giant snake?"

"Bloody hell. . . . Of course Slytherin's monster would be an enormous snake."

"Harry, this explains why you were the only one who heard the voice, it's because you're a parselmouth! But I've never heard of a snake that petrified people--I need to go to the library--" she looks back and forth between Cam and the door, torn between two information sources.

"...do you also have a guess why nobody has seen the snake, because whatever a Parselmouth is I don't think I am one and I can see this, but maybe I have failed to conjure its invisibility gland, how should I know..."

"Well, all the attacks have been on people who were alone, but if it's not invisible whoever's controlling it would have to have a really comprehensive knowledge of the castle to get it around without passing anyone. Can I get a copy of Magickal Serpentes of the Known World?"

"I want to know where it is now," says the redheaded kid.

Magickal Serpentes in the left hand. Snek and environs in the right hand.

The girl grabs the book and grins like Christmas just got rescheduled to every Tuesday.

In the other hand: he gets the whole snek this time, and wow has it got some fangs on it. Also neon yellow eyes. The environs looks for a moment like it might be a big metal pipe that intersects itself at several points, like a nineties video game with bad hitbox programming, then the fact that metal pipe walls aren't meant to occupy the same place simultaneously makes it fall to bits.

"...well, that's... interesting," says Cam, looking at the bits of pipe. "Do you have any... Klein plumbing... around? It interpolated itself!"

"The Hogwarts hallways don't always make sense. You can go down a lot of flat corridors and wind up a floor higher. I guess the plumbing is the same way. And if it's in the pipes, that would explain why nobody's seen it."

"How the blazes is it meant to have entered the plumbing? It's huge! Even if Colin's a kindergartener it's huge!"

"Colin's a first year, he's eleven. And it's supposed to have been hiding somewhere called the Chamber Of Secrets, maybe it's got a way to get something big in the pipes?"

"The insides of the Hogwarts pipes expand to accommodate anything trying to move through them, so they can't get blocked. I read about it in Hogwarts, a History."

"I suppose that's one way to avoid needing to do competent non-Klein civil engineering. Side question. Why is it called 'Hogwarts'. You're speaking English, it can't have escaped you."

"Yeah, why is it called that?"