It gets ugly...
The movie uses a framing device where a man reads to his friends a story from a book by Edgar Allan Poe. He’s a placeholder for the unnamed narrator in the original tale. Not unlike frequent adaptations of Poe’s work, The Fall of the House of Usher is about an estate, about illness, and about ugliness. This house isn’t quite haunted, but that’s how it’s presented.
You sometimes have to tie the dots with Poe adaptations, either because they take shortcuts, or detours, or because they’re restrained by budget, era, running time, technology, and, ultimately, because they don’t translate word for word into more modern feature films. His writing leaves a lot to the imagination because it is stylistic and poetic, and because he was a product of his time.
If the screenwriters took this story literally, there would be a dragon in it. I’m not sure a 1948 picture could handle a dragon. It’s been replaced, at the best of my knowledge, by a hag who’s creepy as hell and offers a few legitimate jump scares. All in all, like most horror films of its time, The Fall of the House of Usher prides itself on its architecture, ambiance, and photography.