Castor and Polydeuces (sometimes called "Pollux"), were in Greek mythology the twin sons of Leda and Zeus and the brothers of Helen of Troy. They are called the Dioscuri (dios kouroi), meaning the "Sons of Zeus." Because Zeus seduced Leda while in the form of a swan, they are sometimes said to have been born from an egg. Polydeuces was a powerful boxer, and Castor a great horseman.
When Theseus and Pirithous kidnapped their sister Helen and carried her off to Aphidnae, the twins rescued her and counter-abducted Theseus' mother, Aethra. Later, they accompanied Jason on the Argo; during the voyage, Polydeuces killed King Amycus in a boxing match.
Castor and Polydeuces abducted and married Phoebe and Hilaeira, the daughters of Leucippus. In return, Idas and Lynceus, nephews of Leucippus (or rival suitors), killed Castor. Pollux was granted immortality by Zeus, and further persuaded Zeus to share his gift with Castor. (On some accounts, only Pollux was fathered by Zeus, while Leda and her husband Tyndareus conceived Castor. This explains why only Pollux was granted immortality.) Accordingly, the two spend alternate days as gods on Olympus and as deceased mortals in Hades.