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...