The author of the Book of Daniel was in error in describing Darius as the earliest King of Persia and as the king who conquered the Babylonians.
The Persian kings are called Achaemenid, as they claimed descent from a legendary King Achaemenes of around the eighth century BCE. The Persians expanded their power by first conquering, then absorbing their Median kinsmen.
The early kings, beginning with Cyrus:
Cyrus the Great (c 560/559-530BCE) was the king who defeated the Median king, Astyages (c 585 - 550), and afterwards conquered the Lydians and the Babylonians, making Persia a great power.
Cambyses (530-522)
Bardia (also known as Smerdis) 522
Darius I (522-486)
Xerxis I (486-465), the king who attempted to invade Greece
Artaxerxes I (465-424)
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Xerxes was king of Persia 486-465 BCE. He and his wife,
Amestris, are known to have had two sons, Darius and Artaxerxes,
but a third son is also mentioned by Diodorus - Hystaspes. There is
no reliable record of any daughters.