Why in 300 were they called the immortals?
They were know as Immortals, not only because they were exceptionally good fighters (ignore 300's depiction, they were noraml Persian men in robes with a long spear and large wicker shield) but also because whenever one died, another instantly took his place. the Immortals were always kept at 10,000 men, so it seemed that none had died after each battle.
Hi, I guess you are you are referring to the Persian Immortals (the ones you can see in the movie 300). They were the elite fighting force of ancient Persia. "The Achaemenid Persian Immortals, usually referred to as the Persian Immortals or The Immortals were an elite force of Persian soldiers who performed the dual roles of both Imperial Guard and standing army during the Achaemenid Empire's expansion and during the Greco-Persian Wars. Herodotus describes…
We know of the Immortals from the writings of Herodotus. Herodotus described a unit of heavy infantry that numbered exactly 10,000 at all times. The name refers more to the unit than the individual soldiers, due to the policy of immediately replacing injured, ill, or deceased members to maintain the 10,000.