In pace requiescat is a Latin phrase which means "May she/he rest in peace." In Poe's classic short story, Montresor vows to get revenge on Fortunato for a "thousand injuries" and elaborates on how he planned and executed the perfect murder. Montresor goes on to reveal how he manipulated Fortunato to follow him into the depths of his family's catacombs, where he eventually shackled Fortunato to the back wall and proceeded to bury him alive. After Montresor forces the last stone in place, he mentions that no mortal has disturbed the bones for a half a century and ends his narrative by saying, "In pace requiescat!" (Poe, 5).
One could interpret the traditional Latin blessing in two ways. Readers could infer either that Montresor made the statement sarcastically or that he sincerely meant what he said. Given the fact that Montresor feels remorse while burying his enemy alive, which is revealed when he confesses that his "heart grew sick," one could infer that Montresor sincerely meant the Latin blessing. The phrase also corresponds to Montresor's feelings of closure. Now that Montresor has got revenge and successfully carried out his murder, he no longer has any ill feelings towards Fortunato, which influences him to repeat the Latin blessing.