Ephialtes of Trachis Ephialtes of Trachis ( , " Ephialtēs " ; although Herodotus spelled it as , " 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 . Trail . 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 . Others . 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 . " The battle . Persian advance . 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 . Last stand . 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 Thespian s , 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 .