Introduction
Zahhak is a tyrant king of Ferdowsi's Shahnameh, whose thousand-year reign of terror, following his usurpation of Jamshid's throne, is one of the epic's central images of monstrous, illegitimate power. His Avestan predecessor, the three-headed dragon Azhi Dahaka, appears in Zoroastrian scripture as a demonic figure defeated in myth by the hero Thraetaona, the model for the Shahnameh's own Fereydun.
The serpent shoulders
According to Ferdowsi, Angra Mainyu, in the disguise of a cook, gained Zahhak's trust and, in a final act of temptation, kissed him on each shoulder. From these kisses grew two black serpents, which could not be removed; when cut off, they grew back. Angra Mainyu, again in disguise, advised that the only way to ease their pain was to feed them human brains each day, condemning Zahhak's kingdom to a horrifying, ongoing sacrifice of its own people.
Usurping Jamshid
With the aid of Angra Mainyu's schemes and Jamshid's own loss of legitimacy through pride, Zahhak seized the Iranian throne, eventually capturing and, in most versions, killing Jamshid by having him sawn in two, and ruled as a tyrant for a thousand years.
Overthrow by Fereydun
Zahhak's oppressive rule provoked the blacksmith Kaveh's popular uprising, which joined forces with the rightful heir Fereydun. Rather than killing Zahhak outright, Fereydun chained him inside Mount Damavand, where, according to the Shahnameh's own prophecy, he remains bound until the end of the world, when he is fated to break free for a final catastrophic episode before the world's ultimate renewal.
Avestan Azhi Dahaka
The Avesta's own dragon-figure Azhi Dahaka, closely related in name and concept to Zahhak, is defeated by the hero Thraetaona (Fereydun's Avestan counterpart) in a similar myth, one of several places where the later Shahnameh draws directly on far older Zoroastrian mythological material, reworked into an extended dynastic epic.