Leonidas I
On the first day of the Battle of Thermopylae, Hydarnes led the Immortals against the phalanx of Spartans under Leonidas I, but an attempt to break through failed. On the second day, a local resident named Ephialtes betrayed the Greeks by telling the Persians about a hidden goat path around Thermopylae. This enabled Hydarnes and his Immortals to pass behind the Spartans, Thespians and Thebans and, as a result, defeat them.