The Persian elite unit known as the Immortals was traditionally said to number 10,000 soldiers. Herodotus described them as a highly disciplined corps that maintained its exact strength through immediate replacement of casualties.
The unit served the Achaemenid kings as elite heavy infantry. Immortals carried spears, swords or daggers, bows, and wicker shields, and wore protective scale armor beneath colorful robes. At Thermopylae in 480 BCE, they were commanded by Hydarnes and were sent against the Greek defenders holding the narrow pass.
Their name may reflect the replacement system rather than literal immortality: whenever a soldier died, became seriously ill, or was incapacitated, another took his place. This kept the corps at a constant nominal strength. Some historians question whether the unit always numbered exactly 10,000, since much of the surviving description comes from Greek sources and later interpretations.
The Immortals were not the entire Persian army, nor were they supernatural warriors as popular films sometimes suggest. They were one prestigious formation within a large, multiethnic imperial military.