Ahriman (Angra Mainyu in the Avestan language) is the supreme evil principle in Zoroastrianism — the ancient Iranian religion founded by the prophet Zarathustra (Zoroaster) that may be the most influential religion most people have never heard of. Ahriman is not a fallen angel, not a former servant of God who rebelled, not a lesser being who exceeded his proper place: he is an eternal, uncreated principle of darkness, chaos and destruction, the absolute opposite and equal antagonist of Ahura Mazda (the Wise Lord, the supreme good deity). This is the world's first fully articulated cosmic dualism — the idea that good and evil are eternal, co-equal, opposing principles — and it shaped the devil-concept of Judaism, Christianity and Islam more profoundly than almost any other theological idea in history.
Zoroaster (dating disputed: between 1500-600 BCE) taught that the cosmos is the arena of a conflict between Ahura Mazda (Truth, Light, Life, Order — asha) and Angra Mainyu (Lie, Darkness, Death, Chaos — druj). These two principles are uncreated and co-eternal — neither derives from the other. The material world is the battleground; human beings are moral agents who must choose which principle to serve; the outcome of history is the eventual triumph of Ahura Mazda and the destruction of Angra Mainyu.
Ahriman's destructive activity is systematic: every good thing that Ahura Mazda creates, Ahriman counters-creates an evil version. Ahura Mazda creates life; Ahriman creates death. Ahura Mazda creates health; Ahriman creates disease. Ahura Mazda creates the bull (the sacred domestic animal); Ahriman creates the predator that kills it. The world as experienced — beautiful and terrible, life-giving and death-dealing — is in this framework the overlapping product of two opposing creative principles rather than the creation of a single all-good God who somehow permitted evil.
The influence on Judaism, Christianity and Islam: the Jewish community spent decades in Babylonian exile (586-539 BCE) and then lived under Persian rule for two centuries (539-334 BCE) — a period of extended, intimate contact with Zoroastrian theology. Scholars of Second Temple Judaism have traced significant Zoroastrian influence on the development of Jewish apocalypticism, angelology, demonology and the developing concept of Satan as an independent evil principle. The sharp dualism of good vs evil, the idea of a cosmic war between light and darkness, the expectation of a final judgment at the end of history, the bodily resurrection of the dead — all of these concepts, central to late Jewish and early Christian theology, have clear parallels in Zoroastrianism and appear in Jewish texts primarily after the Persian period. This does not mean Jewish theology is derived from Zoroastrianism — it means the two traditions were in dialogue, and the dialogue left marks.
Ahriman commands an army of evil spirits called the daēvas (singular daēva) — a word cognate with the Sanskrit deva (god) and the Latin deus (god). This etymological connection is not accidental: the daēvas of Zoroastrianism are the gods of the older Indo-Iranian religious tradition that Zoroaster rejected and demonised. What the older tradition worshipped as gods (the daēvas), Zoroaster redefined as demons. The process of one religion's gods becoming the next religion's demons is visible here with particular clarity.
The specific daēvas include Aēshma (wrath — cognate with the Hebrew Asmodeus), Akōman (evil thought), Nanghaithya (discontent) and many others, each representing a specific vice or destructive force. Together they constitute Ahriman's court — the demonic parallel to Ahura Mazda's divine assembly of the Amesha Spentas (the Holy Immortals who embody divine virtues).