<P> The Immortals (from the Greek Ἀθάνατοι Athánatoi) also known as the Persian Immortals or Persian Warriors was the name given by Herodotus to an elite heavily - armed infantry unit of 10,000 soldiers in the army of the Achaemenid Empire . This force performed the dual roles of both Imperial Guard and standing army . The force consisted mainly of Persians, but also included Medes and Elamites . Essential questions regarding the unit (even its native name) remain unanswered, because authentic sources are missing . </P> <P> Herodotus describes the' Immortals' as being heavy infantry, led by Hydarnes, that were kept constantly at a strength of exactly 10,000 men . He stated 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 corps as a cohesive entity with a constant strength . </P> <P> This elite corps is only called the' Immortals' in sources based on Herodotus . There is evidence of the existence of a permanent corps from Persian sources, which provided a backbone for the tribal levies who made up the bulk of the Achaemenid armies . These do not however record the name of "Immortals". It is suggested that Herodotus' informant has confused the word anûšiya - ("companion") with anauša - ("immortal"), but this theory has been criticized by Rudiger Schmidt . </P>

The deadliest fighting force in all of asia