What is the theme in Claudius the God and His Wife Messalina by Robert Graves?

Fatalism is a theme in this book. The Romans believed that the Fates had already determined their futures, which could not be altered. Claudius describes a visit to the Sybil of Cumae, who foretells of his becoming emperor. He also describes how the Roman Senate would order consultations with the books of prophecies whenever strange portents or disasters occur. Tiberius consults Thrasyllus and acts in response to the soothsayer's prophecies. Livia, near her own death, calls Claudius to tell him of the omens that point to him both becoming emperor and eventually avenging Caligula's death.