After Zeus kidnapped Europa, Agenor ordered her brothers -- Cadmus, Phoenix, and Cilix -- to search for her, instructing them not to return until they had found her. It was a hopeless quest, and all three brothers became exiles. Phoenix founded the country of Phoenicia, and Cilix established Cilicia.
Cadmus consulted the oracle at Delphi and was told to follow a cow that he would find near the oracle; where the cow lay down to rest, he should found a city. He followed the cow to the future site of Thebes. There he instructed his men to bring water so that he could offer a sacrifice to Athena; however, the men encountered a giant serpent which was sacred to Ares, and they were all killed. Cadmus came upon the carnage and gave battle, eventually slaying the serpent. A voice then spoke to him, prophesying that he himself would eventually become a serpent.
Cadmus was left with a site for a city, but no one to help build it. Athene intervened, telling him to sow the serpent's teeth in the earth. He did so, and armed men sprang up from the teeth. They fought one another until only five were left; these five became the ancestors of the noble Thebans. Cadmus then spent eight years in servitude to Ares, as a penalty for the killing of the serpent.
Afterwards, Cadmus married Harmonia, daughter of Ares and Aphrodite. The couple had four daughters (Ino, Semele, Autonoe, and Agave) and one son (Polydorus). Near the ends of their lives, Cadmus and Harmonia left Thebes and went to Illyria. There they were transformed into serpents, as the voice had foretold.