Final answer:
Montresor chained Fortunato to the wall at the extremity of the niche and then entombed him alive by building a wall to seal off the entrance. This was the culmination of Montresor's calculated plan of revenge, using Fortunato's trust and intoxication to trap him in the catacombs forever.
Step-by-step explanation:
When Fortunato reached the extremity of the niche, Montresor swiftly chained him to the granite wall. Montresor took advantage of Fortunato's intoxication and bewilderment to fetter him with a chain that was affixed to the wall by iron staples. Fortunato was too astounded to resist as Montresor secured the padlock and stepped away, effectively beginning the dark act of immurement.
Montresor’s actions were a premeditated part of his plan to seek revenge against Fortunato. He lured his unsuspecting victim into the catacombs under the pretense of verifying the authenticity of a cask of Amontillado wine. Once Fortunato was securely chained and incapacitated, Montresor began to wall up the entrance to the niche, effectively entombing Fortunato alive.
The chilling finality of Montresor's deeds became apparent as Fortunato sobered up. He initially cried out and attempted to pull on the chains, but eventually fell silent. Full of satisfaction, Montresor listened to Fortunato's futile struggles before proceeding to complete the wall, leaving Fortunato to a gruesome fate.