Polycrates did not long survive. He was the first Greek to conceive the idea of a maritime empire. He was foully murdered by the Persian Oroetes, who decoyed him to the mainland by an offer of treasure and then crucified him. In the retinue of Polycrates was a physician, Democedes of Croton, who was captured by Oroetes. His fame spread to Susa at a time when no court doctor could treat Darius' sprained foot. Democedes was sent for and effected the cure; later he healed the Queen Atossa of a boil. Instructed by him she advised Darius to send a commission of fifteen Persians to spy out the Greek mainland under Democedes' guidance. After an exciting series of adventures the physician succeeded in returning to his native city. But the idea of an invasion of Greece had settled on Darius' mind. First, however, he took Samos, giving it to Syloson, Polycrates' brother who years before in Egypt had made him a present of a scarlet cloak while he was a mere guardsman. Darius consolidated his power in Asia by the capture of the revolted province of Babylon through the self-sacrifice of Zopyrus, son of one of the seven conspirators. The vivid story of his devotion is one of the very greatest things in Herodotus. Persia being thus mistress of all Asia, of Samos and the seaboard, began to dream of subduing Greece itself. But first Darius determined to conquer his non-Greek neighbours. The fourth book describes the attack which Darius himself led against the Scythians in revenge for the twenty-eight years' slavery they inflicted on the Medes. A description of Scythia is relieved by an account of the circumnavigation of Africa by the Phoenicians and the voyage of Scylax down the Indus and along the coast of Africa to Egypt.