What does Poe mean when he speaks of the "descent of the soul into Hades" in "The Pit and the Pendulum"?

This phrase comes at the end of a very long sentence that reads as follows:

The thought came gently and stealthily, and it seemed long before it attained full appreciation; but just as my spirit came at length properly to feel and entertain it, the figures of the judges vanished, as if magically, from before me; the tall candles sank into nothingness; their flames went out utterly; the blackness of darkness supervened; all sensations appeared swallowed up in a mad rushing descent as of the soul into Hades.

The narrator, who is in a dreamlike state, has just been condemned to death. The death sentence throws him into a trancelike state, and he describes moving from a strong sense of horror to having a vision of the seven tall white candles before him becoming angels who will save him. Then the vision fades, and it occurs to him that death would be welcome as "sweet rest." Then he describes how the room turns black as he faints—right after he speaks of the "descent as of the soul into Hades," he explains that he swooned.
So, on a literal level, he is describing what it feels like to faint. Fainting feels, right before he loses consciousness, as if he is descending into the underworld. Hades is an allusion to the Greek underworld, a dark, shadowy place where the souls of the dead go.