Ephialtes of Trachis (Greek: Ἐφιάλτης, Ephialtēs; although Herodotus spelled it as Ἐπιάλτης, Epialtes) was the son of Eurydemus of Malis. He showed the Persian forces a path around the allied Greek position at the pass of Thermopylae, which helped them win the Battle of Thermopylae in 480 BC.
The allied Greek land forces, which Herodotus states numbered no more than 4,200 men, had chosen Thermopylae to block the advance of the vastly numerically superior Persian army. Although this gap between the Trachinian Cliffs and the Malian Gulf was only "wide enough for a single carriage", it could be bypassed by a trail which led over the mountains south of Thermopylae and joined the main road behind the Greek position. Herodotus notes that this trail was well-known to the locals, who had used it in the past for raiding the neighbouring Phocians.
Herodotus notes that two other men were accused of betraying this trail to the Persians: Onetas, a native of Carystus and son of Phanagoras; and Corydallus, a native of Anticyra. Nevertheless, he argues Ephialtes was the one who revealed this trail because "the deputies of the Greeks, the Pylagorae, who must have had the best means for ascertaining the truth, did not offer the reward on the heads of Onetas and Corydallus, but for that of Ephialtes of Trachis." 
Led by Hydarnes, a detachment of the Persian army advanced along this path, encountering 1,000 Phocians stationed to block this route. Thinking that they were engaging the entire Persian army whose aim was to attack their nearby homes in Phocis, the Phocians withdrew to defend their city-state, which allowed the Persians to continue along the trail and flank the allied Greeks. The news reached the Greeks at Thermopylae either late that day or before the dawn of the next day, who held a council to decide their next step.
Herodotus is somewhat unclear about exactly what happened next. He provides one account that some of the Greek detachments began to depart for their home towns, while others pledged, despite this development, to stand by the Spartan King Leonidas; he also reports that Leonidas ordered the rest to return home, while the Spartans (who numbered slightly under 300) would stay as a rear guard. The Spartans were joined by about 700 Thespians, who fought to the death beside the Spartans, and the Theban detachment, whom Leonidas held as hostages and who deserted to the Persians at their first opportunity.
Bounty and death
Ephialtes expected to be rewarded by the Persians, but this came to nothing when they were defeated at the Battle of Salamis. He then fled to Thessaly; the Amphictyons at Pylae had offered a reward for his death. According to Herodotus he was killed for an apparently unrelated reason by Athenades of Trachis, around 470 BCE; but the Spartans rewarded Athenades all the same.