Ahriman is the Middle Persian name for Arimanes or Aŋra Mainiiu, a figure from Zoroastrianism, that great Lie (druj) who opposes the truth (arta, asha) of Ohrmazd. Pathfinder brought him in as the master to the Div race.
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".
|The Fiends of Pathfinder|
|Lawful:||Asuras - Devils - Rakshasas - Velstracs|
|Neutral:||Daemons - Divs - Sahkils|
|Chaotic:||Demodands - Demons - Qlippoth|
|Lords:||Ahriman - Archdevils |
Demon Lords - Four Horsemen