"I wonder why some of us were worse affected than others."
"It's not like Sherry's a happy person most of the time," says Tony.
"Yeah. I am though - but then there's whatever's wrong with me that makes me not able to cast a Patronus, I guess."
"Where's your mom? I want to ask her if she knows if there's a way to kill them."
"Oh. I guess I'll have to figure it out, then. Because it is not okay for them to be."
Bella sits down with her. "Where did it come from? Do they just run wild? Do British wizards seriously use those things as prison guards?"
"Yeah. I mean, there's not supposed to be a lot of them running around, but - they can fly, and you can't kill them, and they're hard to trace until they start draining people..."
"How hard to trace? I wonder if Feral can find them for me once I know how to kill them," Bella muses. "He can find stuff when he's changed."
"...I have no idea if anybody's ever tried getting on a Thestral and saying, 'Find me the nearest Dementor'," says Tony. "I know I wouldn't. Those things are nasty."
Bella heads to the household bookshelves and looks for books on dementors.
The available knowledge looks something like this:
Dementors are highly dangerous Dark creatures. They cannot be killed, but may be driven off with the use of a strong Patronus. Under observation they have been known to bud new Dementors, but there is some evidence that they also just appear spontaneously in the wild. They take the form of a corpselike figure in a tattered black cloak with a deep hood. They have some ability to recognize individuals; the ones in use at the British prison Azkaban can accurately tell one prisoner from another. They can fly, not as fast as a good broom but much faster than a running human. Chocolate and a good rest are the best known treatment for brief exposure; with enough of those, persons who have been in the company of Dementors for up to a month have been known to make a full recovery.
Also, they can suck out your soul. There is no treatment for that. The Dementor's Kiss is still widely used in Europe as the ultimate death sentence, but has been banned in the Americas for more than a century.
She looks for more general information on destroying Dark things.
How about theory on how Patronuses work? Her book where she found the spell was not so much about that part.
Bella goes and gets her notebook. She makes lists of things to check on when she has access to the school library again.
And then she looks for something that isn't dessert to eat. She's gotten kind of tired of chocolate.