Zarathustra was born in Bactria (or Aria) as the third of five brothers, and became a priest. After thirty years of praying and fasting, Ahuramazda granted him a vision, which made him oppose the bloody sacrifices of the traditional Iranian cults. Our sources mention that he had an almost fanatical dislike of the priests of the god Mithras, but this may be a late invention. He was forced to leave his country but obtained asylum from a Chorasmian king named Hystaspes.
From now on, Zarathustra preached that there were only two gods, the "wise lord" Ahuramazda and his eternal rival Ahriman. It was the duty of the believer to side with Ahuramazda, which was possible by (a.o.) avoiding lies. The prophet debated with the priests of Mithras; on an official gathering, they discussed thirty three questions, and after Zarthustra's victory, several noblemen -e.g., Hystaspes- became adherents of the new religion. From now on, Zarathustra lived at the court of the Chorasmian king, until he was killed at the age of seventy seven by invading nomads.
Ahuramazdism was the first religion that made ethical demands; its holy book is the Avesta, which contains songs that are probably written by the prophet himself.