Angra Mainyu (also: Aŋra Mainiiu) is the Avestan-language name of Zoroastrianism's hypostasis of the "destructive spirit". The Middle Persian equivalent is Ahriman (Anglicised pronunciation: /ˈɑːrɪmən/).
Avestan angra mainyu "seems to have been an original conception of Zoroaster's." In the Gathas, which are the oldest texts of Zoroastrianism and are attributed to the prophet himself, angra mainyu is not yet a proper name. In the one instance in these hymns where the two words appear together, the concept spoken of is that of a mainyu ("mind", or "spirit" etc.) that is angra ("destructive", "inhibitive", "malign" etc, of which a manifestation can be anger). In this single instance—in Yasna 45.2—the "more bounteous of the spirits twain" declares angra mainyu to be its "absolute antithesis".
A similar statement occurs in Yasna 30.3, where the antithesis is however aka mainyu, aka being the Avestan language word for "evil". Hence, aka mainyu is the "evil spirit" or "evil mind" or "evil thought," as contrasted with spenta mainyu, the "bounteous spirit" with which Ahura Mazda conceived of creation, which then "was".
The aka mainyu epithet recurs in Yasna 32.5, when the principle is identified with the daevas that deceive humankind and themselves. While in later Zoroastrianism, the daevas are demons, this is not yet evident in the Gathas: Zoroaster stated that the daevas are "wrong gods" or "false gods" that are to be rejected, but they are not yet demons.
In Yasna 32.3, these daevas are identified as the offspring, not of Angra Mainyu, but of akem manah, "evil thinking". A few verses earlier it is however the daebaaman, "deceiver"—not otherwise identified but "probably Angra Mainyu"—who induces the daevas to choose achistem manah—"worst thinking." In Yasna 32.13, the abode of the wicked is not the abode of Angra Mainyu, but the abode of the same "worst thinking". "One would have expected [Angra Mainyu] to reign in hell, since he had created 'death and how, at the end, the worst existence shall be for the deceitful' (Y. 30.4)."