In the Italian romance I Due Tristani, the Duke of Calabria is a powerful nobleman who is an ally of King Juan of Castile. He is described as wealthy, influential, and ambitious, with designs on marrying Maria, the daughter of King Juan, in order to strengthen his own position at court.
The Duke sees Tristan the Younger as a rival for Maria’s hand, and he is determined to remove him from the picture. He conspires against Tristan and tries to have him killed, both by arranging a treacherous ambush during a tournament and by launching a full-scale military attack on him and Maria after they flee to Brittany.
The Duke is portrayed as a ruthless and cunning antagonist, willing to use any means necessary to achieve his goals. His character represents the classic literary trope of the villain who opposes the hero’s love and seeks to thwart their happiness.
The Duke of Calabra is not unique to I Due Tristani, he appears in other versions of the Tristan and Iseult (Isolde of Cornwall) legend as well. In some versions, he is known as the Duke of Cornwall or the Duke of Morgan, depending on the location and culture in which the story is told.
Source
I Due Tristani | 1555