Poe's utilizes elevated diction (a formal, polysyllabic style). Note the references below for literary terms definitions.
Poe uses a variety of literary devices in his diction. Poe uses irony in “The Cask of Amontillado.” The irony that Poe employs foreshadows the imminent death of Fortunato. In Italian Fortunato means “Fortunate” but it is clear to see that Fortunato was unfortunate at the end of the story. Irony is also used in reference to Fortunato’s cough. Montresor tells Fortunato that he should not go into the vault with his cough and Fortunato tells Montressor that his cough isn't going to kill him. Montresor tells him that's true because he knows he is plotting his death by burying him alive behind a wall of stone. Visual irony is also evident when Fortunato wants proof that Montresor is a mason and part of the Mason Secret Society. Montresor shows Fortunato his trowel, which indicates his knowledge of construction with stone. This foreshadows Montresor's planning of Fortunato's entombment and eventual death. Finally, Poe utilizes irony when Montresor keeps asking Fortunato if he would like to turn back. Even though Fortunato is terribly drunk and stumbling, he is determined to keep going. Montresor's final statement to Fortunato, before he delivers his death sentence, being chained to the wall is, “Once more let me implore you to return." If Fortunato hadn't been so bound and determined to make his trek through the vault, there is a possibility that he wouldn't have met such a terrible fate.
Poe's diction in the story which is told from the first person point of view is that of an unreliable narrator. Montressor, who is the character that tells the story, uses exaggerations in his diction (or word choice). For example the story opens with Montressor claiming, "The thousand injuries of Fortunato I had borne as best I could. . ." is an obvious exaggeration and right away leads the reader to believe that there may be some bias in the story about to be told, not to mention that the reader should now question the credibility of the narrator from here on out. Other examples of unreliable diction are the plots for revenge and references to Fortunato as his "poor friend" (which we know is untrue from Montressor's point of view).