Herodotus describes a corps of ten thousand picked Persians whose number was kept permanently at strength, every casualty replaced at once, and says they were called the Immortals for that reason. They form the royal guard and appear at Thermopylae as the troops sent in when others have failed.
The name is a problem. A widely discussed suggestion is that the Greek athanatoi, 'immortals', renders an Old Persian word closer to 'companions' or 'followers', anusiya, and that Herodotus or his informant misheard or reinterpreted it. The proposal is not proven and not universally accepted.
As a military formation the Immortals are thinly evidenced: they are described by an enemy, and they do not appear in the Persian record under a corresponding name. What is well attested is the existence and prominence of royal guard troops, visible in the glazed brick panels from Susa showing spearmen in patterned robes.
What the evidence supports
Herodotus describes a 10,000-strong guard kept permanently at strength.
The account is clear and is the origin of everything later said about the unit.
The name may be a mistranslation.
The proposed Old Persian original, anusiya, would mean something like 'companions'. The argument is philological and remains contested.
Royal guard troops are attested in Persian art.
The Susa archer panels, now in the Louvre and elsewhere, show guardsmen with spears, bows and quivers in elaborately patterned dress.
Archaeology
The glazed brick archer frieze from the palace of Darius at Susa is the principal visual evidence. It is royal self-representation and its relation to any specific unit is an interpretation.
Open questions
Were the Immortals a distinct standing corps?
Herodotus's account is the only substantial one, it comes from an enemy tradition, and the Persian administrative record does not obviously corroborate it. That a royal guard existed is not in doubt; the specific institution as Herodotus describes it is less secure than its fame suggests.
Primary sources
Herodotus, Histories 7.83, 7.211
The description of the corps and its use at Thermopylae.
Battles
480 BCE
Thermopylae
A Greek force held a coastal pass in central Greece for three days against the army of Xerxes, and was destroyed when the position was turned by a mountain path.
479 BCE
Plataea
The largest hoplite battle of the Persian Wars, which destroyed the Persian field army in Greece and ended the invasion.
331 BCE
Gaugamela
The decisive battle of the Persian war, fought on ground Darius had chosen and prepared, and after which the Achaemenid empire had no field army left.
How to read the evidence labels
- Evidence level: Documented
- Supported by a named primary text or by excavated material evidence.
- Evidence level: Probable
- Supported by strong inference that most specialists accept.
- Evidence level: Disputed
- Substantial specialist disagreement exists; competing readings are stated.
- Evidence level: Literary
- Belongs to poetic or mythological construction rather than to history.
- Evidence level: Mythological
- Belongs to shared religious or mythic tradition rather than to any single text.
- Evidence level: Unknown
- The available evidence is insufficient to decide.
These labels describe the status of the evidence, not our confidence in a conclusion. A claim marked Literary is not a claim we doubt; it is a claim about what kind of thing a poem is.