What was a new religion that developed during Darius's rule of the Persian Empire?

Darius adopted the religion promoted by Zoroaster (Zarathustra), who simplified the Persian pantheon of gods into just two opposing ones - Ahura Mazda (Illuminating Wisdom) and Angra Mainyu (Destructive Spirit).

Zoroastrianism still has about a million adherents in Persia, Pakistan, India, and scattered through the world, with the US home to the second largest number. In India they are known as Parsees.