Much like his opposite, Ormazd, the perceived history of Ahriman holds that Ahriman has existed since before time was time. These two opposite beings have remained the same for countless ages, locked in a constant battle of light and darkness. Whereas Ormazd dwells high above even other gods of other lands, Ahriman dwells far below all known existence. There are those who speculate he lives somewhere in the Underworld, or that he is in fact all darkness within the Underworld, and there are those who hold that he lives in the utter blackness of Ginnungagap. Ahriman takes many shapes and bears many names across every land in the mortal realm, for darkness lives everywhere. One such example is Isfet, thought to be his moniker in Kemhet.
Ahriman fully embodies everything evil and wrong, including literal darkness, the opposite of light and good. All wicked acts thus pay homage to Ahriman: the corruption of anything good, destruction, death, theft, and every other imaginable sin. Hence, there are those who believe Ahriman may be some kind of demon or, at least, a god that reigns over even all the demon lords themselves. Ahriman's followers claim he exists everywhere, in all shadows that inhabit every land and every realm. They say the more darkness that exists, the more Ahriman himself grows in power.
The common people of Parsanshar do not "worship" Ahriman: they acknowledge his existence and power - and the power that Ormazd holds over him, which is why it is necessary to uphold Ormazd and aid him in his battle against darkness. However, Ahriman still has his own direct followers, as well. Those who worship Ahriman must do so in secret, living in clandestine cults or worshiping alone in privacy. His followers would uphold corruption and wickedness, as well as literal darkness itself.