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:

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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.