Ephialtes of Trachis

Ephialtes (/ˌɛfiˈæltiːz/; Greek: Ἐφιάλτης, Ephialtēs; although Herodotus spelled it as Ἐπιάλτης, Epialtes) was the son of Eurydemus (Greek: Εὐρύδημος) of Malis. He betrayed his homeland, in hope of receiving some kind of reward from the Persians, by showing the army of Xerxes a path around the allied Greek position at the pass of Thermopylae, which helped them win the Battle of Thermopylae in 480 BC.