King of Sparta from 491 to 480 B.C.; opposed Xerxes, the Persian, who threatened Greece with a large army, and kept him at bay at the Pass of Thermopylæ with 300 Spartans and 5000 auxiliaries till he was betrayed by Ephialtes (q. v .), when he and his 300 threw themselves valiantly on the large host, and perished fighting to the last man.