The Cavalcantis are imposters, whom the Count of Monte Cristo is using to get revenge against Villefort.  Major Cavalcanti is supposed to be the wealthy and noble Italian father of “Count” or “Prince” Andrea Cavalcanti.  Andrea is played by the bastard son of Villefort and Madame Danglars, Benedetto.  Monte Cristo introduces them to Paris society, and Andrea succeeds in getting engaged to Eugenie Danglars.  Andrea is blackmailed by Caderousse  and kills him, and is then arrested for the murder. During Andrea’s trial, he reveals his true identity and thereby ruins Villefort and drives him insane.