The intensity of the darkness seemed to oppress and stifle me. The atmosphere was intolerably close. I still lay quietly, and made effort to exercise my reason. I brought to mind the inquisitorial proceedings, and attempted from that point to deduce my real condition.
The first person, unnamed, narrator in Poe’s "The Pit and the Pendulum" heightens the horror of the story by adding to the story's validity. Even as the narrator tells his seemingly implausible story of imprisonment, torture, and rescue, the use of the first person adds to the story by allowing the reader to go through the trial and imagine life in the prison through his eyes. Just as if you hear a story first hand from a friend, the reader to gets the story directly from the man himself. In short, the first person narrator allows the reader to imagine they are in on the story from the survivor of the horrific tale.
The use of first person also helps to heighten the horror in "The Pit and the Pendulum" because of its limitations. Because the reader is seeing the events unfold through the eyes of a frightened man who must figure out where he is and how to escape, the reader is only able to know what the man knows and see what he sees. That means any time the man is fumbling around in the dark, the reader is right there with him. When the narrator discovers that he avoided a gaping pit by mere inches in the dark, the jolt of surprise and the sense of relief is genuine in the reader, because both the narrator and the reader find out about this brush with death at the same time. Such revelations on the part of the narrator would not have nearly the same effect if the story were told in, say, third person omniscient. "There is a pit in the room, but the narrator doesn't know it. He stumbles next to it and misses it by inches. Later, he discovered he missed it." The reader is set in a far more detached, observational position when you take out the first person perspective. There is a consistent sense of urgency and tension in the forced limited view, and the mystery is kept intact so the reader can experience the narrator's same fear of the unknown.