Poseidon is one of Zeus's brothers, and the ruler of the seas.
A long time ago, before mortals existed, the Titans ruled the world. However, Kronos, King of the Titans, was warned of a prophecy where one of his six children would kill him and take control of the world from him. In response, Kronos began swallowing his children in hopes of preventing the prophecy from coming true. Poseidon was devoured by Kronos after his brother Hades was devoured. However, Poseidon's mother, Rhea, saved the final child Zeus by tricking Kronos into swallowing a stone wrapped in a blanket while an eagle carried the real Zeus to safety. Poseidon and Hades grew up insde of their father's stomach while Zeus hid from Kronos. Years later, Poseidon was freed by Zeus when he gave Kronos a potion which made him vomit Poseidon and his siblings as well as the stone substituted for Zeus. After being freed, Poseidon went to the Underworld with Zeus and freed the cyclopes, who gave Poseidon a trident as a gift. Later, Zeus, Poseidon, and Hades declared a ten year war on the Titans called the Titanomachy. During the Titanomachy, Hades, created a giant monster called the Kraken to help them against the Titans. Finally, after ten years of war, Zeus,Poseidon, and Hades finally defeated Kronos and banished their father and the Titans to Tartarus where they hoped they would remain imprisoned forever. Poseidon became God of the Sea, Zeus became God of the Sky and the King of the Gods, and Hades became God of the Underworld. Zeus, as his first act as ruler of the world, created mortals to populate it. Years later, Poseidon had an affair with an unnamed mortal woman and had a son named Agenor around the same time Zeus had an affair with a female mortal named Danae and had a son named Perseus.