Name one detail that creates a mood of suspense or horror, then describe how this detail creates its effect.

Practically everything that Poe writes in "The Cask of Amontillado" is concerned with creating an atmosphere of suspense. One such example is the way he contrasts the bright fun of the carnival with the gradual darkness that Montresor and Fortunato descend into as they go deeper and deeper into the catacombs. The darkness of the atmosphere reflects the increasingly dark mood of the story as Fortunato is brought nearer to his doom.
Yet it all began so brightly! The carnival is a time of joy and celebration, when people get together to have fun and enjoy themselves. Fortunato has entered into the spirit of things, dressing up in a jester's costume and getting more than a little tipsy. As a master storyteller, Poe realizes that it's always more effective to start off bright and go dark, instead of diving straight into the darkness. Such an approach gives the writer more time to develop the story, building up an appropriate mood of suspense, so that we as readers are as suckered into Montresor's dastardly trap as much as the hapless Fortunato.