Hydarnes son Hydarnes was an eminent Persian who was placed as commander of the 10,000 Immortals. The Immortals got their because if anyone in their ranks was killed or fell sick they would be replaced. Hydarnes lead the Immortals in Xerxes I expedition to invaded Greece. He lead the Immortals with Elphialtes as a guide on the their surprise attack from the rear on the 300 Spartans, 700 Thespians and 400 Thebans under King Leonidas of Sparta. The Spartans and their allies were trapped and massacred. After the Persian naval defeat to a much smaller Greek fleet Xerxes and most of his remaining army. (except the 300,000 men who stayed behind under the command of Xerxes' cousin, Mardonius to try to subdue Greece and were defeated at the battle of Platea) Xerxes gave Hydarnes the responsibility of getting the army back over the Hellespont and back to Asia.