What literary device is used in "The Cask of Amontillado" how does it develop the message of the story?

Though his dark subject matter is not universally appealing, Edgar Allan Poe was a superb technician and his skill in crafting this revenge tale through the use of symbolism is undeniable.
A single example among the many uses of symbolism is found in the names of the principal characters, Montresor ( French for "my treasure") and Fortunato (ltalian for "fortunate one").  Montresor is pathologically angry with Fortunato for a series of slights that are described only as "a thousand injuries". Exacting revenge through Fortunato's destruction is what is most precious to Montresor.
Fortunato, symbolically costumed as a fool for Carnival, is merrily oblivious to Montresor's anger and readily follows him underground to sample a rare sherry. When Montresor overpowers and entombs Fortunato, he literally buries his "treasure." Fortunato's name, then, can only be deemed ironic.
 Poe, Edgar Allan.  "The Cask of Amontillado"
http://xroads.virginia.edu/~hyper/poe/cask.html