In "The Pit and the Pendulum," what has the unknown narrator dreamed?
I take it you are referring to the dream that the unnamed narrator experiences after being condemned to death at the beginning of the story. It is interesting to note that, before we are told what he actually dreamed about, Poe builds suspense by making readers wait to learn what happens next as the narrator gives us a meditation on dreams and consciousness. Finally, however, we are told that he remembers the following sequence of events after he regained consciousness:
These shadows of memory tell, indistinctly, of tall figures that lifted and bore me in silence down--down--still down--till a hideous dizziness oppressed me at the mere idea of the interminableness of the descent. They tell also of a vague horror at my heart, on account of that heart's unnatural stillness. Then comes a sense of sudden motionlessness throughout all things...
Note how the dream the narrator recalls vaguely could be interpreted symbolically as a descent into the grave, into hell or into despair as we picture silent figures bearing an inert person, whose heart is still, deep into the earth. This of course foreshadows the terror and despair that the narrator has yet to experience.