Why, the ultimate revenge, of course. At least that is Montressor's intention. He lures Fortunato into the catacombs and walls him up there. Motressor thinks that murder is the revenge he seeks; however, this act alone doesn't fully satisfy Montressor's need for vengeance. When Fortunato stops crying out or begging for mercy, Montressor taunts his foe, trying to get him to scream or beg, but instead he is answered with silence except for the jingling of bells on Fortunato's hat. See the excerpt below:
"No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick - on account of the dampness of the catacombs."
Notice how Montressor excuses his rushing in the end by saying the dampness of the catacombs made his heart grow sick. But an astute reader will notice the dash-which makes the excuse of the dampness seem almost an afterthought-as if he is trying to convince himself that it was the dampness, and not his own disappointment in Fortunato's reaction, or lack thereof, that made his heart sick. So, even though Montressor's plan is a success, it fails to completely satisfy him. He does not get the ultimate thrill of hearing his enemy shriek in horror or beg for mercy. That is what Montressor was hoping for. So his victory over Fortunato is less fulfilling than he had hoped. Fortunato steals his glory by not reacting to Montressor's cruelty.
He walls him up in the catacombs of Amontillado and leaves him there to die. He has lured his victim deep within the underground wine storage area and tricks him into exploring it further. Here is the conclusion of Poe's horrifying tale:
No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick - on account of the dampness of the catacombs. I hastened to make an end of my labor. I forced the last stone into its position ; I plastered it up. Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them. In pace requiescat !
It is never disclosed, however, why Montressor is so upset with Fortunado. He refers vaguely to a history of insults, and knows the man is a wine snob and a bore, but other than that, the charges are not clear.