Persian kings, including Xerxes, subsequently developed a core of professional soldiers who owed allegiance only to the king. The most famous of these professionals were dubbed by the Greeks to be the "Immortals" because, the ranks of these warriors never seemed to diminish in number. In fact, the Immortals were a unit of 10,000 highly-trained and skilled infantrymen. Xerxes' great army numbered in six figures including the Immortals and defeated the Greeks at Thermopylae.
Alexander the Great eventually conquered the Persian Empire, folding it into his short-lived Macedonian Empire.