Ahriman's physical manifestation is your horny beasty black balrog, complete with whip; but with vulture-taloned feet. Under his skin, snakes undulate eternally.
Ahriman was born with the first djinn, in their shadow; Chaos' own reaction to the Divine work of creation. So he is theologically apart from the rebel-angel myths of Islam and late-antique Christendom.
His domains are Darkness, Death, Destruction, and Evil - but especially Destruction, since he's not a daemon. His sigil is (per the third Bestiary) "a black circle with a thin sliver of light overtaken by darkness".