The tone is serious. When Fortunato thinks that it is all a joke and requests to be gone, Montressor comments back "Yes, let us be gone." referring to Fortunato's death. Montressor had a very serious tone throughout the entire book.
I know exactly how you will die.
Ugh Ugh Ugh Ugh
Although Fortunato is dressed as a joyous clown like figure, his outfit greatly contrasts with the fate that is awaiting him.
The vaults are insufferably wet.
Montressor wants to toast to Fortunato's long life. In actuality, he is toasting to his inevitable death.
Fortunato tells Montressor that his cough won't kill him. Montressor replies "True-true.", knowing exactly how he will die.
The narrator uses imagery to describe the cold and damp walls of the catacombs. "The vaults are insufferably damp."
The family arms of a foot crushing a snake and the snake is biting back. The is foreshadowing what will happen to Fortunato.