Angra Mainyu — "the Hostile Spirit," or, in the later Middle Persian, Ahriman — was the cosmic adversary of Ahura Mazda, the source of all evil in the world, the chief opponent of all that is true and right. He was not a created being (so the priestly tradition insisted, against the temptation of Zurvanism that we shall meet below) but a principle equally primal with the Holy Spirit Spenta Mainyu — though always inferior to Ahura Mazda in being and knowledge. He had emerged at the beginning of the world from a certain darkness, and his first act had been to attack the good creation; from that attack came every disease, every death, every winter, every drought, every poison, every lie.

The Bundahishn — the Pahlavi cosmogonic text — describes Angra Mainyu's first irruption into the world in terms of horror. He came in the form of a great serpent (or as a fly, or as a black smoke); he pierced the sky, polluted the water, scorched the earth, killed the first plant, killed the first ox, and made his attack on Gayomard the first man. From his attack came death — for before Angra Mainyu's irruption, nothing had died — and from death came every other evil. The world that we know, in which all things suffer and die, is the world contaminated by his presence; the world Ahura Mazda had originally made was wholly good and wholly immortal.

But the Hostile Spirit's attack was also his trap. By entering the world to attack it, he had committed himself to a struggle in which he could not in the end prevail. Ahura Mazda had foreseen exactly this: the long war was the means by which Angra Mainyu would be exhausted, weakened, and at last defeated. The whole present age — the age in which we live — is the period of the war, called the period of "mixture," in which good and evil are tangled together in every thing and every person. At the end of the age, the Saoshyant will come, the dead will rise, the final battle will be fought, and Angra Mainyu will be cast down forever. After that comes the Frashokereti, the renewal of all things, in which the world is restored to the wholly good condition Ahura Mazda originally intended.

Until that day, the Hostile Spirit must be opposed by every righteous person, in every act of life. To eat, to wash, to sleep, to pray, to till the field, to keep the fire on the hearth — these are all small contributions to the great cosmic war on the side of Truth. The Iranian believer was a soldier in a war that had no neutral ground; he must take a side, and the side he took determined his eternal destiny.