Ahriman

Here's my vision of Ahriman (or Angra Mainyu, or Asra Manyu), the Persian god of evil, darkness, and destruction, generally opposed to his antithesis Ahura Mazda (or Ohrmazd), the good creator god of light. However, their relationship, nature and origins tend to vary from one ancient Iranian religion to another. In Mazdaism, a Persian religion that would predate Zoroastrianism and yet is generally considered the same (according to several authors like Martin Haug, Jean Varenne, Abel Hovelacque, and Ludovic Richer), Ahriman actually opposes Spenta Mainyu, the good spirit (who himself is often equated with Ahura Mazda), and both are emanations of Ahura Mazda.
In Zurvanism, a branch of Mazdaism supposedly born during the second half of the Achaemenid dynasty's rule (between 486 and 330 BC, thus), Ahriman opposes Ahura Mazda, and both are twins born from Zurvan, the supreme deity of infinite time. Still following the previously named scholars' analysis, while Ahriman is an actual god in these doctrines, he becomes an abstract concept in actual Zoroastrianism : the absence of the virtues promoted by Zoroaster and Ahura Mazda (who himself became a much more abstract entity in this religion). But still, having a good understanding of these religions' links and differences remains very difficult. Finally, in Manichaeism (a religion founded by the Parthian prophet Mani in the 3rd century AD, as a contestant to Zoroastrianism), the Prince of Darkness takes the name of Ahriman, opposed to the Father of Greatness, who takes Zurvan's name.

I'll now tell more about Ahriman himself, according to several Yasnas of the Avesta, the Bundahishn (around the 9th-12th century AD), the Shahnameh (around 1000 AD), and the Mazdaist vision globally, in which Ahura Mazda creates the world, and remains more powerful than the evil god. Indeed, while Ahura Mazda's power goes beyond the material world, Ahriman's is bound to it. But the evil god still managed to create death, diseases, hunger, vices, and many demonic servants called daevas (or divs). Among these were the druj, female demons ruled by Jahi (or Jeh), the demoness of lasciviousness, consort of Ahriman himself. Ahriman is also told to have created winter, all biting, crawling and stinging animals, as well as all scavenging creatures, flies, etc... The evil god almost dried up the earth twice, urged Zoroaster to join his side by promising him sovereignty over the world before being rejected.
At one time, Ahura Mazda created Gavaevodata (or Gawi Ewdad), the primordial hermaphrodite ox that gave birth to all benevolent animals, taking advantage of Ahriman's 3000 years prostration state due to a previous defeat. Upon his "awakening", the evil god slaughtered the divine animal, taking the form of a monstrous wildcat. Ahriman also created the three-headed dragon Azhi Dahaka, which was chained under Mount Damavand but will unleash at the end of time (a parallel version of this peculiar story exists in the Shahnameh, were the evil god takes various human forms to corrupt King Zahhak and make him one of his servants, until he met the same fate as the dragon).
A simple contact with Ahriman can have dramatic consequences : by simply kissing King Zahhak's shoulders under a human form, the evil god made two bloodthirsty serpent heads sprout from the man's body, and by copulating with Jahi after she motivated him to walk against Ahura Mazda once again after 3000 years, women accross the world started to experience menstrual cycles. Ahriman also seems to be able to corrupt and/or slay living things just with his gaze, his "evil eye" so to speak.