The Immortals (from the Greek Ἀθάνατοι, sometimes Ten Thousand Immortals or the Persian Immortals) was the name given by Herodotus to an elite heavy 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.