Half-Olympian son of Zeus by Leda; heteropaternal twin of mortal Castor (by Tyndareus). One of the Dioscuri, the divine twin-pair central to Spartan and Roman cult. Master boxer; killed Amycus on the Argonautic expedition. Refused full immortality without Castor when his brother died in the Apharidae feud; Zeus permitted shared alternating immortality between Hades and Olympus, eventually catasterizing both as the constellation Gemini. The Roman Temple of Castor and Pollux in the Forum was vowed at Lake Regillus 496 BCE.