Ephialtes revealed the mountain route that allowed the Persians to outflank the Greek defenders at Thermopylae. He was a Greek from Trachis who informed Xerxes’s army about a trail running over the mountains behind the Greek position.
The Greeks knew that the narrow pass could be bypassed, so Leonidas had stationed Phocian soldiers to guard the route. Ephialtes guided the Persian commander Hydarnes and his troops along it during the night. The Phocians were surprised or withdrew, allowing the Persians to descend behind the main Greek force.
Leonidas then dismissed most of the allied army and remained with a rear guard, including the 300 Spartans and 700 Thespians. Ephialtes expected a reward from Xerxes, but he received nothing and later fled. A Greek bounty was placed on him, and according to Herodotus he was eventually killed by Athenades of Trachis around 470 BC. The name Ephialtes later became associated with treachery in Greek tradition.