Maybe there's some sort of plot or secret that he doesn't want Hermione to expose. He might think that by keeping her close, he can prevent her from telling others. For example, if he's involved in some sort of illegal or unethical magic practice and he believes Hermione has figured it out.
It's also possible that other characters would get involved. Harry and Ron, being Hermione's closest friends, would surely notice her absence. They could come to her rescue. There might be an epic battle of magic between them and Draco, or they could try to reason with Draco, appealing to his better nature if he has one.
She could also start to negotiate with Draco. Given her intelligence, Hermione might try to understand Draco's motives and then use reason to persuade him to let her go. She might offer alternatives or compromises that would satisfy both of their needs.
It could also be related to some sort of family or pure - blood related matter. Draco's family might have some connection to Hermione that he has just discovered, and he's trying to keep her in place to sort out the implications of this connection, which might involve protecting her from other dark forces or just making sure she doesn't interfere with his family's plans.
He might seek help from other characters in the fanfic. Maybe there is a wise old man or a helpful friend who could persuade his mother or give him advice on how to convince her to let him leave. Another option could be for Ash to find a compromise. Maybe he can suggest a short - term leave first to test the waters, rather than a permanent departure.
Hermione could leave Draco if she realizes that they have fundamental differences in values. For example, if Draco still holds some pure - blood supremacist views, Hermione, being a Muggle - born advocate, would not be able to tolerate it in the long run.
Well, perhaps the werewolf is attracted to Hermione for some special reason. Maybe it's her magical power or her kind heart. In the fanfiction, it could lead to a series of dangerous situations for Hermione, like she has to constantly be on guard against the werewolf's unexpected approaches.
There's also a chance that it's a werewolf from another wizarding community that has come to Hogwarts and has fixated on Hermione. This werewolf could be on a mission, perhaps sent by a dark wizard, and Hermione just happens to be in his path, and he won't leave her alone as part of his orders.
It's fiction. In real life, there are no houses with a will of their own that can physically prevent kids from leaving. Such a concept is often used in fictional stories to create a sense of mystery and danger. For example, in some horror - themed children's literature, a haunted house might be described as having this strange power to trap the young protagonists. This serves to engage the readers' emotions and make them curious about how the kids will escape.