How did Ephialtes help the Persian army defeat the Greeks at Thermopylae?

During the Battle of Thermopylae in 480 BCE, the Greek forces successfully held a narrow pass against the massive Persian army for two days, effectively neutralizing the Persians' numerical advantage.12

The stalemate was broken by a Greek citizen named Ephialtes, who betrayed his countrymen in hopes of receiving a reward. Ephialtes informed the Persian king Xerxes I of a mountain path that bypassed the main Greek defensive line.21

Guided by Ephialtes, a detachment of elite Persian infantry known as the Immortals utilized this path to descend behind the Greek position. This maneuver outflanked the Greeks, rendering their defensive line useless and forcing King Leonidas to dismiss the majority of his troops to ensure their safety. Leonidas and a small contingent remained to fight to the death, delaying the Persian advance into southern Greece.123

It is important to note that this individual, the traitor Ephialtes, is a different historical figure than the Athenian politician of the same name who later championed democratic reforms.4