The rulers of an empire stretching from Libya to Central Asia in the 5th, 4th and 3rd Centuries BCE.
Persian.
King of Kings.
The Persian and Mesopotamian kings and the Roman Emperors were all absolute rulers.
Darius and Xerxes.
A monarchy.
They did.
The Achaemenid kings. See the Website below.
On google.
Succession.
Their kings and councils.
The commanders appointed by the Persian kings and by the Greek city-states.
There were several Persian leaders called Darius - they were kings of Persia.