King Leonidas and the Greeks under his command kept the vastly superior Persian army bottled up in front of the pass at Thermopylae until Ephialtes struck. He informed King Xerxes that he knew of a track through the mountains that bypassed Thermopylae, and reemerged to join the road behind the Greek position. In exchange for the promise of rich rewards, Ephialtes showed the Persians the way. When he discovered that he was about to be outflanked, Leonidas sent his Greek allies away.
He stayed behind with what remained of a 300-strong contingent of Spartans, who fought to the death until they were wiped out. Ephialtes was reviled from the ancient era to the present, and his name came to mean “nightmare” in Greek. He never collected his reward: the Persian invasion collapsed when their fleet was defeated at Salamis later that year, and their army was crushed at Platea the following year. Ephialtes fled, with a bounty on his head. He was killed ten years later over an unrelated matter, but the Spartans rewarded his killer anyhow.
Where Did We Find This Stuff? Some Sources and Further Reading