The tone of the story is overwrought and unrelievedly dark and fevered, reflecting the oppressive, foreboding setting and unstable, hypersensitive psyche of Roderick Usher.
As the narrator rides toward the house, he notes a sensation of melancholy and gloom which he calls
insufferable; for the feeling was unrelieved by any of that half-pleasurable, because poetic, sentiment with which the mind usually receives even the sternest natural images of the desolate or terrible. ... the bitter lapse into every-day life—the hideous dropping off of the veil. There was an iciness, a sinking, a sickening of the heart—an unredeemed dreariness of thought which no goading of the imagination could torture into aught of the sublime.
This is an important passage, because here, (through the voice of his narrator), Poe, who is believed to have borrowed the story's opening passage from Radcliffe's The Mysteries of Udolpho, is from the start setting straight any misconceptions the reader might take away from the allusion to Udolpho: the mood of the story will be relentlessly grim, not shot with the sublime like Radcliffe's novel. In the late 18th and early 19th century, the sublime was a popular form, deriving, as the narrator so well describes it, "half-pleasurable," often awe-inspiring and uplifting feelings from "desolate" or scary places. This story, Poe is signaling, will not contain any uplift, and it does not.
The unrelenting tone of darkness continues as the narrator arrives at the house. Roderick has a "ghastly pallor" and, overtaken by nerves and a sense of horror, is given to "wild improvisations" on his guitar, painting grim pictures, and working with a "wild ritual." The tone of horror grows as the story progresses to its frightening and macabre end.