According to Greek myth, Cadmus' descendants ruled at Thebes on-and-off for several generations, including the time of the Trojan War. For a discussion of the mythical kings of Thebes, see Theban kings - Greek mythology.
After his sister Europa had been carried off by Zeus from the shores of Phoenicia, Cadmus was sent out by his mother to find her, enjoined not to return without her. Unsuccessful in his search, he came to Samothrace, the island sacred to the "Great Gods and the Kabeiroi, whose mysteries would be celebrated also at Thebes. At Samothrace, Cadmus was not journeying alone: he appeared with his "far-shining" mother Telephassa in the company of his brother, who gave his name to the island of Thasos nearby. An identically-composed trio had other names at Samothrace, according to Diodorus Siculus: Elektra and her two sons, Dardanos and Eetion or Iasion. There was a fourth figure, Elektra's daughter, Harmonia. whom Cadmus took away as a bride, as Zeus had absconded with Europa. The wedding was the first celebrated on earth to which the gods brought gifts, according to Diodorus and dined with Cadmus and his bride.
The cow was given to Cadmus by Pelagon, King of Phocis, and it guided him to Boeotia, where he founded the city of Thebes. Robert Graves (The Greek Myths) suggested that the cow was actually turned loose within a moderately confined space, and that where she lay down, a temple to the moon-goddess (Selene) was erected: "A cow's strategic and commercial sensibilities are not well developed," Graves remarked.
Intending to sacrifice the cow to Athena, Cadmus sent some of his companions to the nearby Castalian Spring, for water. They were slain by the spring's guardian water-dragon (compare the Lernaean Hydra), which was in turn destroyed by Cadmus, the duty of a culture hero of the new order.
By the instructions of Athena, he sowed the Dragon's teeth in the ground, from which there sprang a race of fierce armed men, called Spartes ("sown"). By throwing a stone among them, Cadmus caused them to fall upon one another until only five survived, who assisted him to build the Cadmeia or citadel of Thebes, and became the founders of the noblest families of that city.
The dragon had been sacred to Ares, so the god made Cadmus to do penance for eight years by serving him. According to Theban tellings, it was at the expiration of this period that the gods gave him Harmonia as wife. At Thebes, Cadmus and Harmonia began a dynasty with a son Polydorus, and four daughters, Agave, Autonoë, Ino and Semele.
At the wedding, whether celebrated at Samothrace or at Thebes, all the gods were present; Harmonia received as bridal gifts a peplos worked by Athena and a necklace made by Hephaestus. This necklace, commonly referred to as the Necklace of Harmonia, brought misfortune to all who possessed it. Notwithstanding the divinely ordained nature of his marriage and his kingdom, Cadmus lived to regret both: his family was overtaken by grievous misfortunes, and his city by civil unrest. Cadmus finally abdicated in favor of his grandson Pentheus, and retired with Harmonia to Illyria, whose inhabitants proclaimed him their king and founded the city of Lychnidos and Bouthoe.
Nevertheless, Cadmus was deeply troubled by the ill-fortune which clung to him as a result of his having killed the sacred dragon, and one day he remarked that if the gods were so enamoured of the life of a serpent, he might as well wish that life for himself. Immediately he began to grow scales and change in form. Harmonia, seeing the transformation, thereupon begged the gods to share her husband's fate, and she did (Hyginus).
In another telling of the story, the bodies of Cadmus and his wife were changed after their deaths; the serpents watched their tomb while their souls were translated to the fields. In Euripides' The Bacchae, Cadmus is depicted as being turned into a dragon, or alternatively a serpent, after Dionysus overthrows Thebes.