The opening paragraph should make us feel like something creepy is going on for sure. First of all, the narrator begins by saying that he’s nervous and assuring us that he’s not crazy.
"The disease had sharpened my senses—not destroyed—not dulled them."
Already we have a sense of disease, decay and worry. This tells us this might be a spooky story.
If that did not give it away, the following sentence certainly would.
Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell.
By focusing specifically on hell, we again get the impression that this is likely to be a horror story. The narrator says he is going to “calmly” tell us the whole story, another indication that it might be a frightening story