Ephialtes was the son of Eurydemus of Malis. He betrayed the Spartan king Leonidas at the Battle of Thermopylae in 480 BC, by helping the Persian king Xerxes I find another route around the pass of Thermopylae. This allowed the Persians to defeat the small band of Spartan defenders, none of whom survived.
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, with a bounty on his head. According to Herodotus he was killed for an apparently unrelated reason by Athenades of Trachis, around 479 BC.
Another Ephialtes was leader, with Pericles, of the radical democratic movement in Athens. In 462 BC Ephialtes reformed the Areopagus, limiting its power to judging cases of homicide and religious crimes. This was an unpopular move and he was assassinated in 461 BC.