Pollux was a demigod and a son of Zeus. His twin brother was Castor, who was born to Leda and Tyndareus, the king of Sparta.
Their mother was Leda, but Castor was the mortal son of Tyndareus, the king of Sparta, and Pollux the divine son of Zeus.
Castor was killed, Pollux asked Zeus to let him share his own immortality with his twin to keep them together, and they were transformed into the constellation Gemini. The pair was regarded as the patrons of sailors, to whom they appeared as St. Elmo's fire, and were also associated with horsemanship.
During the expedition of the Argonauts, Pollux took part in a boxing contest and defeated King Amycus of the Bebryces, a savage mythical people in Bithynia. After returning from the voyage, the Dioskouroi helped Jason and Peleus to destroy the city of Iolcus in revenge for the treachery of its king Pelias.
When their sister Helen was abducted by the legendary Greek king Theseus, the brothers invaded his kingdom of Attica to rescue her. In revenge they abducted Theseus' mother Aethra and took her to Sparta while setting his rival, Menestheus, on the throne of Athens. Aethra was then forced to become Helen's slave. She was ultimately returned to her home by her grandsons Demophon and Acamas after the fall of Troy.