The Magi held significant religious and political power during the Babylonian World Empire, the Medo- Persia Empire, the Greek Empire, and during the Roman Empire. In all of those empires they maintained a place of tremendous prominence and political power. They have been advisors to kings and emperors throughout the ancient Middle East for astrology, policy, soothsaying, dream interpretation and religious issues.
The Magi were so powerful that no Persian was ever able to become king except under two conditions: One, he mastered the scientific and religious discipline of the Magi. Two, he had to be approved of and crowned by the Magi. Not only did the Magi advise the king, they choose the king and the king had to obey the laws of the Magi. The Magi get to decide who is king.
After some Magi attached to the Median court proved to be expert in the interpretation of dreams Darius the Great established them over the state religion of Persia. It was in this dual capacity whereby civil and political counsel was invested with religious authority, that the Magi became the supreme priestly caste of the Persian Empire, and continued to be prominent during the subsequent Seleucid, Parthian, and Sasanian periods.