From Wikipedia, the free encyclopedia: Persian Immortals, detail from the archers' frieze in Darius' palace in Susa. Silicious glazed bricks, c. 510 BC. The "Immortals" (from the Greek Ἀθάνατοι, sometimes "Ten Thousand Immortals" or "Persian Immortals" In Persian سپاه جاودان) was the name given by Herodotus to an elite force of soldiers who fought for the Achaemenid Empire. This force performed the dual roles of both Imperial Guard and standing army during the Persian Empire's expansion and during the Greco-Persian Wars. Its Persian name may have been Anûšiya ('companions'). Herodotus describes the 'Immortals' as being heavy infantry led by Hydarnes that were kept constantly at a strength of exactly 10,000 men. He claimed that the unit's name stemmed from the custom that every killed, seriously wounded or sick member was immediately replaced with a new one, maintaining the cohesion of the unit.