Ahriman, being the Middle Persian equivalent of Angra Mainyu (meaning "absolute antithesis"), is the chief evil spirit in Zoroastrianism. Comparable to the Christian concept of the Devil, or Satan. He's the antithesis of the god Ahura Mazda, who instead represent the chief deity of the Zoroastrian pantheon. In some novels, he is depicted as an avenger god.
Ahriman is the Demonic Sponsor of Hikawa, promoting the Reason of Shijima. He is summoned within the heights of the Diet Building, as a member of the Tyrant Race. If the Demi-Fiend's ideals differ from Shijima, he must fight and kill Ahriman to continue past a certain point in the Tower of Kagutsuchi. If the Demi-Fiend follows Shijima himself, Ahriman lets him pass without conflict, giving him the Netherstone key item as a token of good will. If fought, Ahriman is introduced as a peculiar boss, in that demands to be "amused" before killing the protagonist. He will randomly forbid the use of specific actions, such as physical attacks, items usage, demon summoning, and so on, killing instantly whichever character failed to respect these limitations with his signature move Hell's Call.
However, after a certain amount of damage, he will lose himself to the bloodlust and deploy his full offensive capabilities, releasing his tentacles from their bindings and using them as whips and piercing weapons. During the second phase, he will lose the ability to restrict the enemy's actions, but will instead gain a destructive Almighty-type move, Apocalypse. When Ahriman dies, at the top of its head, Hikawa will acknowledge his defeat and regret his vision of the world of Shijima would not come to pass, passing into the silent realm of the dead.
Angra Mainyu is the final boss of the Neutral route of the game. After clearing both the Amnesia and Paranoia maps, you will be sent back to Megapolis where you will engage a large number of demons and humans under his control. Angra Mainyu himself is extremely weak to Ice, so several Bufu spells or ice element related skills should finish him off rather quickly. It is suggested that no demons who fly face him due to the Vile race's affinity that makes them extremely effective against aerial foes.
Angra Mainu appears as an enemy in the Extra Dungeon as one of the highest leveled demons in the game. He can only be encountered in class 3-A.
In the additional PSP scenario, Angra Mainu resides in the Tower of Ethics that is part of the Kadath Mandala. He is surprised to see humans in this world and recalls that 25940 days have passed since he last met Randolph Carter. He asks the party if there is there such a thing as unwavering goodness. He approves of all their answers and tells them that goodness and wickedness are relative concepts, not limitless. Nevertheless, rivalry is necessary to explain things that are connected. If he, the one who owns the Trapezohedron of Ethics is evil, then they too, the ones who wish for it, are evil. Throughout history the destruction of antagonistic peoples and countries has been encouraged in the name of gods as a good thing. Is this a virtue?
Katsuya answers his question correctly: If there is no good and evil then Angra Mainyu would not exist. Thus without the boundary between the two neither could Ahura Mazda. He then gives them the Trapezohedron of Ethics.
Although the actual Angra Mainyu is considered a prototype of God's archenemy and a "god" of chaos, in Majin Tensei II he is a Vile, a Dark-Law race; an odd choice for the being considering this. To add to this irony, in the same game, his arch-enemy, Ahura Mazda, is a Tenma, a Light-Chaos race, which is also odd considering he is a prototype of God and a god of order.