In his essay "The Importance of a Single Effect in a Prose Tale," Poe writes that he unifies a piece of writing around mood. He writes not primarily to develop a plot or a character but to convey a feeling or what he calls an "effect."
Most often in his stories, Poe wishes to convey a mood or "effect" of horror. He does this through description and imaginative details that relentlessly build up a sense of unsettling terror. For example, in "The Cask of Amontillado," the reader's awareness that Montresor is plotting revenge and the piling up of creepy details about the cold, damp, bone-filled catacombs through which he leads Fortunato builds a mounting sense of tension and deep unease. Similarly, the ebony clock that stops everyone cold when it ominously tolls the hour in "The Masque of the Red Death," reminding people of their mortality in the middle of a deadly plague, contributes to a sense of horror.
Poe also tightens his effects by using a claustrophobic writing style focused on very few characters and often narrated by a person who is troubled or unstable. Poe sometimes horrifies us by putting us into contact with a fevered mind trying to justify its heinous actions, as in "The Tell-tale Heart," or with a claustrophobic nightmare setting, such as that described by the first-person narrator of "The Pit and the Pendulum.
