The "Immortals" (Greek: Ἀθάνατοι Athanatoi) is a name used by Roman historians of the Roman-Persian Wars to refer to an elite unit of the army of the Sasanian Empire.
The reported Greek name and the size of the force is identical to the "Immortals" infantry unit of the Achaemenid Empire described by Herodotus.
Arabic sources of the Islamic period also mention elite forces during the Muslim conquest of Persia (for example, that of Jalinus).
[1] According to one report, on one occasion during the Roman–Sasanian War of 421–422, the force attacked a Roman army, and although becoming outnumbered, they stood firm and were killed to the last man.
[1] Although modern historians of the twentieth century had accepted this description and interpreted it as a revival of the Achaemenid unit by the Sasanians, recent reinvestigations have doubted the simplified Roman description, based on the recent view of Touraj Daryaee (2006) (itself based on Ehsan Yarshater's theory) that the Sasanians had little knowledge of the specific institutions of their Achaemenid predecessors.