Hydarnes was the son of another Hydarnes, one of the seven conspirators who had killed the Magian usurper Gaumâta and helped Darius become king of the Achaemenid empire in 521 BCE. The elder Hydarnes became an influential man: from the tablets found at Persepolis, we know that in 499, he was still satrap of Media. This was one of the most important satrapies; only Bactria, Susiana and Babylonia were of equal importance, although Egypt and India were wealthier.
The influence of this elder Hydarnes secured the appointment of his sons as satraps. The Greek researcher Herodotus tells us that in 480, his son Sisamnes was satrap of Aria and that the younger Hydarnes - the subject of this article - was "in command of the whole Asian seaboard". (Maybe, he was already in the west before: according to Herodotus, a Hydarnes served in Thrace c.499.note[Herodotus, Histories 6.133.])
During the Greek war of Darius' son Xerxes (who became king in 486), the younger Hydarnes was commander of the Immortals, which was - according to Herodotus - the name of the Persian elite corps. During the battle of Thermopylae (in September 480), Hydarnes and his men made a nightly detour through the mountains and attacked the Greeks in their rear. They were still in Greece during the next year, when Mardonius continued the war after the setback at Salamis. Hydarnes, however, returned with Xerxes to Asia; the Immortals were defeated at Plataea.
In the fourth quarter of the fifth century, two descendants of Hydarnes were influential in Persia: Tissaphernes became satrap of Lydia and Caria in the years 413-395 and a Hydarnes was the father-in-law of king Artaxerxes II Mnemon. The family fell into disfavor in 395 BCE.
These two noblemen may have been the grandsons of Hydarnes.