One way that Poe is able to build tension in the "Pit in the Pendulum" is by making the narrator of the story the victim and protagonist of the story. It's written in first person, so the reader only knows as much as the narrator. Sadly, the narrator, and by consequence the reader, knows very little. The vagueness and absence of detail is done by having the narrator swooning in and out of a stable state of mind. For example, one minute he sees candles, the next he sees angels, and then everything fades out again.
And then my vision fell upon the seven tall candles upon the table. At first they wore the aspect of charity, and seemed white slender angels who would save me; but then, all at once, there came a most deadly nausea over my spirit, and I felt every fibre in my frame thrill as if I had touched the wire of a galvanic battery, while the angel forms became meaningless spectres, with heads of flame, and I saw that from them there would be no help.
Soon after he passes out. When he finally awakes, he's in a new, unfamiliar location. Have you ever woken up in a new, unfamiliar place? I have, and there is always that moment of panic that rises up inside of me. Then my brain catches up. For the reader, that is what Poe has done by having the narrator wake up in a new location. To make matters worse, the darkness is so complete that it feels tight and oppressive.
At length, with a wild desperation at heart, I quickly unclosed my eyes. My worst thoughts, then, were confirmed. The blackness of eternal night encompassed me. I struggled for breath. The intensity of the darkness seemed to oppress and stifle me. The atmosphere was intolerably close.
I went down into the Carlsbad caverns a few years ago. The tour guide shut off all of the lights at one point. Even though I knew where I was and that the lights were going to come back on in a minute or so, it was still very creepy. The darkness was so complete, that it felt hard to breathe. That's what Poe describes in the above lines of text. The darkness creates tension, because deep down, people are afraid of the dark.
Poe also uses repetition to create tension in the story. He repeats certain words or phrases to build tension in the reader. The best example, I believe, is when the pendulum is working its way down to the narrator.
Down—steadily down it crept. . . Down—certainly, relentlessly down! . . . Down—still unceasingly—still inevitably down!
Every time I read this story, I feel myself beginning to read faster and faster as Poe repeats the downward movement of the pendulum. Part of my brain is always screaming "just get there already!" so that I can know what happens. Poe narrates inches of movement over paragraphs of space, and the tension just builds and builds, because Poe doesn't immediately answer the reader's main question. "Will he live or die?" That sense of unknown is a huge tension builder and Poe is a master at crafting it.