Here is Poe's theory or doctrine about the prose tale briefly stated in his famous review of Nathaniel Hawthorne's Twice-Told Tales, published in Graham's Magazine in May, 1942:
A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents, he then combines such events as may best aid him in establishing this preconceived effect. If his very initial sentence tend not to the outbringing of this effect, then he has failed in his first step. In the whole composition there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design. And by such means, with such care and skill, a picture is at length painted which leaves in the mind of him who contemplates it with a kindred art, a sense of the fullest satisfaction. The idea of the tale has been presented unblemished, because undisturbed; and this is an end unattainable by the novel. Undue brevity is just as exceptionable here as in the poem; but undue length is yet more to be avoided.
By "effect" Poe meant the emotional effect upon the reader, the feeling or mood the reader is left with upon finishing the story. It would be impossible for a creative writer to produce this effect without first feeling it himself.
In Poe's famous story "The Cask of Amontillado" the single effect is experienced by the reader at the end, when Fortunato is left to die in a horrible setting in a horrible manner. But the single effect of a short story does not have to be produced by the ending. In "The Fall of the House of Usher," Poe demonstrates that an effect can be produced by the overall tone and mood of a story. Poe begins to establish the preconceived effect from the "very initial sentence" of his story.
During the whole of a dull, dark, and soundless day in the autumn of the year, when the clouds hung oppressively low in the heavens, I had been passing alone, on horseback, through a singularly dreary tract of country; and at length found myself, as the shades of the evening drew on, within view of the melancholy House of Usher.
Poe understood that a mood can be communicated through a succession of words, mostly adjectives, and his first paragraph is literally crammed with words intended to produce the feeling of gloom, decay, foreboding, dread, hopelessness, and tragedy. Note the many such words in just the first sentence: dull, dark, oppressively, alone, dreary, shades, melancholy. This deliberate use of mood-inducing words and combinations of words continues throughout the story. Here are a few more examples from the first paragraph:
a sense of insufferable gloom
trunks of decayed trees
an utter depression of soul
the hjideous dropping off of the veil
an iciness, a sinking, a sickening of the heart
a black and lurid tarn
the ghastly tree-stems, and the vacant and eye-like windows
Poe's theory about the prose tale applies equally well to "The Fall of The House of Usher" as to "The Cask of Amontillado," although the single preconceived effect in each story is achieved in a different way.