Ephialtes of Trachis
Ephialtes of Trachis (Greek: Ἐφιάλτης, Ephialtēs; although Herodotus spelled it as Ἐπιάλτης, Epialtes) was the son of Eurydemus of Malis. He betrayed his homeland, in hope of receiving some kind of reward from the Persians, by showing 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 that 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 neighboring 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 1000 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. However Plutarch in On the Malice of Herodotus claimed this story of the Thebans is unlikely, and writes the Spartans launched an attack on the Persian Camp that forced Xerxes to flee his tent.
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 BC; but the Spartans rewarded Athenades all the same.
In popular media
In the 1962 film The 300 Spartans, Ephialtes was portrayed by Kieron Moore and is depicted as a loner who worked on a goat farm near Thermopylae. He betrays the Spartans to the Persians out of greed for riches (and, it is implied, unrequited lust for a Spartan girl named Ellas, portrayed by Diane Baker). 
Frank Miller's 1998 comic book miniseries 300, and the 2006 film adaptation of the same name, portray Ephialtes as a severely deformed Spartan exile whose parents fled Sparta to protect him from the infanticide he would have surely otherwise suffered (as a disabled and, therefore, unfit warrior). He is portrayed in the film by British actor Andrew Tiernan. 
- Ever since Ephialtes betrayed the Greeks at Thermopylae, in Greek "ephialtes" means nightmare.
- Ephialtes also is used in Greek as a synonym for traitor, in a comparable fashion to the usage of the words Quisling or Judas in the English language or Benedict Arnold in the US.
- Battle of Traigh Ghruinneart, where a dwarf switches sides and kills the leader of the forces he originally was to fight for
- Macaulay, G.C. "The History of Herodotus". The University of Adelaide. paragraph 213. Retrieved 2007-03-28.
- Persian Wars. p. 213.
- Herodotus, Histories, 7.200
- Herodotus, Histories,7.215
- Herodotus, Histories,7.214
- Herodotus, Histories, 7.218
- Herodotus, Histories,7.222, 7.235
- Herodotus, Histories,7.213