What is the first torture in the "Pit and the Pendulum"?
The first torture could be any number of things, depending on whether you want to classify the torture as only physical or include psychological.
If you include psychological torture, I would say that the first torture is the narrator's sentencing. He begins to break down and swoon the moment he is pronounced guilty. He's crazy with fear at that point, because he knows what's coming, and he essentially starts to hallucinate.
I saw, too, for a few moments of delirious horror, the soft and nearly imperceptible waving of the sable draperies which enwrapped the walls of the apartment. And then my vision fell upon the seven tall candles upon the table. At first they wore the aspect of charity, and seemed white and slender angels who would save me...
After the narrator passes out due to his fear, he wakes up in his torture chamber. Again, if I get to include psychological torture, I could pick this part. He has no idea where he is. He remembers everything with sudden intensity and knows what's coming. He is practically frozen with fear. He can't even bring himself to open his eyes.
I longed, yet dared not to employ my vision. I dreaded the first glance at objects around me. It was not that I feared to look upon things horrible, but that I grew aghast lest there should be nothing to see.
He then opens his eyes and that's another instant torture. Complete and utter darkness. I'm talking about the kind of darkness where you can't see the hand in front of your face. I'm not afraid of the dark, but that kind of blackness is scary and oppressive feeling. And that's how the narrator feels.
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.
After a bit, the narrator calms down and decides to explore his room. After walking around the room, he decides to walk across it. He trips and falls flat just before the pit. That's probably the first torture that most people would list. That makes sense, since it is part of the story's title.