(Canto 12, line 17)
The "Duke of Athens" refers to Theseus, son of Aegeus. He was sent to Crete to deliver the group of children to the Cretan king. Arrived in Crete, Minos' daughter Ariadne (and half-sister to the Minotaur) fell in love with him, and gave him a ball of string to take with him into the Labyrinth so that he wouldn't get lost. In the end, Theseus killed the Minotaur with a sword given to him by Ariadne, rescuing the lives of the fourteen children. After that, he was chosen as the king of Athens.