"Okay, so I've been able to tell from context that's a big deal but Professor Reed hasn't really covered them, help?"
"Dementors are soul-eating monsters who suck the happiness out of anyone who gets too close. And until Miranda killed one, they couldn't be destroyed."
"They could be repelled, with a spell called the Patronus Charm, which is made of happy memories. Most people's looks like an animal made of silver light. Miranda's looks like a person, and instead of shooing Dementors away it chases them down and kills them. She saved our lives - we were looking for Jenny, me and her and Emma and Professor Fisher - and there was a Dementor that had Jenny cornered, and she'd morphed her mouth away so it couldn't take her soul, but she couldn't run away. Professor Fisher fainted. I couldn't do anything, Emma couldn't, but Miranda knew the spell and she'd never gotten it to work before but that time it worked and the Dementor was gone."
(She'd point out that Karen hasn't really answered her question about 'how' but from the sound of it no one, possibly including Miranda, can answer that, so she doesn't bother pressing.)
"No wonder she's famous," she says faintly.
"Yeah. And she keeps doing it! She goes out, summers, to places where there might be dementors, and she camps out and stalks them and kills them. She wants to drive them extinct."
Vivian frowns. "I'm not sure I like the idea of driving anything totally extinct," she admits, "but I am also used to the Muggle world, where all animals are just... animals. And usually necessary to a food chain or something like that. Dark creatures are probably different."
"Well, you'd have to talk to her about keeping one in a menagerie, I guess, because she knocks down a couple dozen every summer. Left wild they do sometimes suck out random people's souls though."
Vivian shrugs. "I don't know anything about them," she reminds Karen. "Sharks kill people sometimes too, but I know they do- something. With fish. And eating fish. Keeping the oceans balanced?" She laughs. "They do something! I'm in magic school, not Muggle school, I missed the lectures on fish."
"You don't... go with her, right?" Vivian has a horrible vision of Karen's soul being sucked out, Karen who doesn't know how to kill a Dementor, and she suspects she's found her nightmare fodder for the next few months.
"Oh no. I'd be completely useless. Gilderoy Lockhart used to escort her but now she has someone from the International Confederation of Wizards, I forget her name, some junior aide who apparates her places and finds more dementors for her to kill."
"Those- Gilderoy Lockhart? International Confederation of Wizards?" Vivian looks sheepish. "I'm sorry, usually I just smile and nod and pretend I know what's happening, but I can actually ask you. I can stop if it's annoying."
"No, it's fine! Gilderoy Lockhart wrote a lot of books about fighting monsters, and he taught Defence one year, and then he had a memory charm accident and was in St. Mungo's for a long time but then he got mostly better, better enough to take Miranda dementor-hunting. Miranda didn't like him very much but no one else would do it at first so she put up with him. And the International Confederation of Wizards is a thing all the magical governments form together, sort of?"
"Most of the things magic can't just fix are caused by magic to begin with. If you, I don't know, lose your arm because a rock fell on it, Madam Pomfrey will fix it up. If you have a potions accident she might not be able to. She probably could, mind, but she might not."
"I wonder how many Muggle problems magic could solve but no one's tried?" Vivian sighs. "I don't really understand the hiding, I guess."
"Muggles often misplace their brains," Vivian says. "But they're over that one. And lots of other horrible things we used to do... mostly." She looks at Karen seriously. "Besides. I saw what the school was like when we started. Two of the girls in Slytherin still wake us up every few months screaming with nightmares. One boy breaks down crying every so often and won't say why. Most of the older students are orphans. It's not like wizards don't struggle with this stuff."
"I thought they got over it because we started hiding. And I'm not sure what there having been a war has to do with witch burnings."