After overthrowing the Titans, the three brothers Poseidon, Zeus and Hades drew lots in order to share justly the powers of the Cosmos.
Zeus price were the heavens and he became the supreme ruler of the Greek gods; Hade's price, however, was a rather displeasing one, since he was chosen by fate to get the Underworld. Poseidon's price, on the other side, were the seas and this way, Poseidon became the lord of the seas and the second in power among the greek deities.