In Greek mythology, the Dioscuri were the twin brothers Castor and Pollux (also called Polydeuces). Their mother was Leda, but they had different fathers. Tyndareus, the king of Sparta, was the father of Castor (hence a mortal), while Zeus was the father of Pollux (a demigod). Some sources say that they were born from an egg, along with their twin sisters Helen and Clytemnestra.
The myth has it that Leda was seduced by Zeus, who had taken the form of a swan. However, there are different versions as to whether the twins were both mortals, both immortals, or Castor was mortal and Pollux was a demigod.
The twins participated in the Argonautic Expedition, helping Jason retrieve the Golden Fleece. Upon their return to Greece, they also aided Jason in taking revenge for the treachery of Pelias, king of Iolcus, by destroying the city. They also took part in the hunt of the Calydonian Boar. Later, when their sister Helen was abducted by Theseus, king of Attica, they attacked his kingdom. They freed their sister and abducted Theseus' mother Aethra in revenge, who served as a slave for Helen.
Castor and Pollux fell in love with Phoebe and Hilaeira, nieces of Leucippus, who were known as the Leucippides and who were already the consorts of the Dioscuri's cousins, Lynceus and Idas. The twins abducted the Leucippides and brought them back to Sparta, thus starting a family feud. The cousins attacked the region of Arcadia and managed to claim a herd of cattle from the Dioscuri, who were enraged. Some time later, the cousins visited their uncle's palace; the Dioscuri, Helen, and Paris were present at the time. The Dioscuri thought it was a good opportunity to take their cattle back and left for their cousins' home. Lynceus and Idas left some time later, leaving Helen and Paris alone, which eventually led to the Trojan War. The cousins caught the Dioscuri while attempting to steal the herd; Castor was mortally wounded by Idas, while Pollux killed Lynceus. As Idas approached to kill Pollux though, Zeus threw a thudnerbolt against him, thus saving his son's life. Pollux then asked his father to grant half of his immortality to his brother. Hence, they transformed into the constellation Gemini, able to travel back and forth between Olympus and Hades.