What in "The Fall of the House of Usher" symbolizes Hell?
The house itself represents a sort of hell, especially for Roderick. He is tortured by his sister's "death" and his torturous condition which causes him to be overly sensitive to any type of sensory stimulation.
Specifically within the house, the vault in which Madeline's "corpse" is placed also symbolizes hell. The narrator observes as he helps Roderick with Madeline's body that
"[it] . . . was small, damp, and entirely without means of admission for light; lying, at great depth. . . . It had been used, apparently, in remote feudal times, for the worst purposes of a donjon-keep [dungeon], and in later days, as a place of deposit for powder, or some other highly combustible substance, as a portion of its floor, and the whole interior of a long archway through which we reached it, were carefully sheathed with copper" (Paragraph 22).
Poe paints a picture of the vault as having a flaming appearance (the copper coating) and being at extreme risk of catching fire. Additionally, the vault was once used for torture, which evokes images from Dante's Inferno's rings of torture.
At the story's end, the house is engulfed in flames as the narrator escapes, and Poe leaves the impression that the underworld or hell swallows up the Usher family and all its associations.