Answer:
i believe it's simply the imagination of those in the house. The narrator never mentioned any specific details of monsters or ghouls. the narrator is contradicting himself in the story so that he may not seem to be as reliable. Sometimes, if a place seems creepy or ominous, your mind begins to play tricks on you, making you hear or see things which aren't actually there. I believe the narrator knew that Madeline was alive at the back of their head and knew the consequences as well and felt weighed by the guilt. he slept in a room right above the vault where she had been buried so he must've heard her struggling and banging on the vault to get out, and instead of checking the vault to see if she were alive, he simply convinced himself that she was a ghost.
Step-by-step explanation: