Angra Mainyu, the Destructive or Hostile Spirit, known in later Zoroastrian tradition as Ahriman, is the principle of evil in Zoroastrianism, the source of death, disease, falsehood and darkness, set against the good creation of Ahura Mazda, the Wise Lord. He is not, however, best understood as a mere devil in the later Western sense.
In the ancient doctrine of the two spirits, set out in the Gathas attributed to the prophet Zarathustra himself, there are two primal Mainyus, two fundamental dispositions or spirits, one that chooses truth and life, and one that chooses the lie and destruction, and evil enters the world through a choice rather than existing as an eternal given equal to the good. This makes Angra Mainyu the embodiment of the wrong choice, the active will toward ruin, and it places moral decision at the very centre of the cosmos: human beings, like the spirits, must align themselves with truth (asha) against the lie (druj).