What was the persians' strong army of highly trained soldiers called

the ten tousand Immortals

Explanation:

These Persian Warrios were famous thanks to the stories by Herodotus. He describes an elite infantry heavly armed, gathering ten tousand soldiers seen as a single unity. At Thermophilae they collided with a minuscule yet heroic Greek defense. The immortals served as Imperial Guard and a all land fighting army. Only Persian or Medes of origin could join the frontlines. Their armor consited of bronze breastplates , with furnished helmets, shoulder protectors. They often carried spears, secondary swords and were able to fight in different settings. Since the regiment was accompanied by caravans of camels and mules, they always secured supplies,  like food and medicines making possible to mantain a force of 10 000 men eager to fight. When a soldier felt in combat, it was inmediatly replaced by its  nearby replacement unit.

This gained the Immortals the name since the perception was that they would never suffer significant losses.

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