What was the name of the elite 10,000-man infantry force in ancient Persia?

The Immortals were an elite infantry force of the Achaemenid Persian army.

The name comes from Greek historians, especially Herodotus, who described a corps of 10,000 soldiers. According to his account, every fallen, sick, or absent member was immediately replaced, keeping the unit’s reported strength constant. This replacement system likely inspired the name “Immortals,” although the Persian designation may have been different.

Herodotus portrays the unit as heavily equipped and closely associated with the royal court. The Immortals formed part of the forces used in major campaigns, including Xerxes’s invasion of Greece. Their presence at Thermopylae is a famous episode in the Greek historical tradition.

The unit should not be confused with the Greek “Ten Thousand,” the mercenary army described in Xenophon’s Anabasis. Nor should every Persian soldier in a royal relief be identified as an Immortal; the corps is known mainly through literary and visual evidence.