What are the major literary devices used in "The Fall of the House of Usher?"
Clearly you have a lot to choose from, as Poe is a master storyteller who uses literary devices well. I am going to approach this question by referring to how Poe uses setting in this chilling story. Whether it is the catacombs in "The Cask of Amontillado" that reflect Montresor's disturbed and twisted character of the House of Usher, whose dilapidation reflects the own mental instability of its owner, Roderick Usher, Poe always uses setting to great effect in his gothic, spine-chilling tales of horror. Consider what we are told about the House of Usher:
Its principal feature seemed to be that of an excessive antiquity. the discoloration of ages had been great. Minute fungi overspread the whole exterior, hanging in a fine tangled web-work from the eaves. Yet all this was apart from any extraordinary dilapidation. No portion of the masonry had fallen; and there appeared to be a wild inconsistency between its still perfect adaptation of parts, and the crumbling condition of the individual stones. In this there was much that reminded me of the specious totality of old woodwork which has rotted for long years in some neglected vault, with no disturbance from the breath of the external air.
Note the overall emphasis on rottenness and decay. The setting of course is a symbol that could be said to represent the madness and mental disturbance at the heart of the owner of the house. At first glance it appears to be of sound quality, but closer examination reveals issues that could indicate serious structural problems. Remember, the narrator has journeyed here, not out of choice, but because Roderick Usher, his childhood friend, has written to him to come and be with him as he is suffering from a "nervous agitation" and a "mental disorder". How mentally disturbed he is will only be revealed at the end of the story...
Therefore the description of the House of Usher is used as a symbol of the madness and dilapidated state of its owner, and thus Poe effectively foreshadows what is to come.