These 10,000 Persian Elite Infantry force fought in many battles from Salamis to Marathon, they were King Darius's body gaurds, they are called...?

The Immortals were the elite 10,000-strong Persian infantry force who served as King Darius's personal bodyguards. They fought in major battles including Marathon, Thermopylae, and Salamis. They were called Immortals because their strength was always maintained at 10,000 - whenever a member died or was wounded, he was immediately replaced.