During their descent into the catacombs, Fortunato and Montresor pause several times to take a drink. Montresor is happy to do so, since he wants to make sure Fortunato is drunk when they reach their final destination. At one point, Fortunato toasts the bodies that "repose" around them. When Fortunato comments about the extensive size of the underground vaults, the narrator responds that
“The Montresors... were a great and numerous family.”
“I forget your arms.”
“A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel.”
“And the motto?”
“Nemo me impune lacessit.”
“Good!” he said.
The Latin motto of the Montresor family translates to "No one attacks me with impunity," impunity meaning "without punishment." Thus, Montresor's act of revenge correlates with the family motto, punishing Fortunato for his own attack--an eye for an eye.
Sorry, SAVLOV338, I posted an answer to another question about the "Cask of Amontillado" to your question.
There are answers to your question about Montresor's coat of arms in the links below in enotes questions. Hope this helps!
In medias res, or “into the middle of things” is Latin phrase used to describe a story that begins in the middle or even at the conclusion instead of at the beginning.
The main character and narrator of the story, Montresor, relates his story of revenge to an unnamed listener, the mysterious “you” who is acquainted with Montresor. He narrates his story in the past tense, referring to events that have already happened.
Finally, the clearest clue that Montresor is looking back and telling the story after the fact comes at the very end with these statements:
I forced the last stone into its position; I plastered it up. Against the new masonry I reerected the old rampart of bones. For the half of a century no mortal has disturbed them.
With the final sentence, we know that Montresor’s revenge happened fifty years ago--and no one has found and disturbed Forunato’s tomb since Montresor buried Fortunato alive.