It all began with Montresor telling how fortunato kept insulting him and he had enough and wanted revenge.
During the carnival in Italy, Montresor runs into Fortunato and offers him a nice bottle of wine. however, this bottle is in his catacombs. It is is late at night and Fortunato seems ill.
Montresor takes Fortunato to his catacomb / wine cellar. Fortunato started coughing uncontrollably and getting sick. Then every time Fortunato coughs he offers him a glass of alcohol because he told him it would keep him from coughing but truly he was trying to get him even more drunk.