Aži Dahāka — "the Snake Dahaka" — was the great cosmic adversary of the Iranian heroic tradition, the three-headed dragon who was the most fearful of all the creatures of Angra Mainyu. He had three mouths and six eyes; he had three heads, on the central neck of which it was difficult to land a killing blow; his body was filled with foul creatures (snakes, scorpions, lizards, toads) that would burst forth and pollute the world if his body were ever opened. He was, in short, the embodiment of cosmic evil in concentrated form, the chief weapon of the Hostile Spirit against the good creation.
The Avestan tradition places his origin in the early days of the world, before even Yima's golden age; he was created by Angra Mainyu specifically to attack the good creation. He sought to extinguish the fires of Atar; he sought to seize the Khvarenah; he sought to destroy the cattle and the cultivators of the earth. After Yima's fall, Aži Dahāka established himself as the tyrant of the world and reigned for a thousand years, slaying Yima at the end of his usurpation by sawing him in two.
His defeat by Thraetaona (treated above) did not kill him but bound him; he is chained beneath Mount Damavand, gnawing eternally at his chain, awaiting the moment of his final release. At the end of the world, the chain will break, Aži Dahāka will burst forth, and he will devour a third of all life on earth — both human beings and animals, and even the stars themselves — before being slain by the eschatological hero (Keresaspa in the older tradition, the Saoshyant in the later) at the final cosmic battle. His death will mark one of the closing acts of the cosmic war, after which the world will be renewed in the Frashokereti.
In the Shahnameh, Aži Dahāka appears as Zahhāk, the great tyrant of the early Pishdadian period. Ferdowsi gives him a more humanized and tragic backstory: Zahhāk was a young Arab prince of pure character, corrupted by the suggestions of Eblis (the devil); he murdered his father, took the throne of Iran by treachery, and after Eblis kissed him on the shoulders, two black serpents grew there which had to be fed daily on the brains of two young men. Zahhāk's tyranny lasted a thousand years before Faridun (= Thraetaona) overthrew him and chained him beneath Damavand. The Shahnameh's Zahhāk is at once a literal historical fiction (a wicked king) and the same cosmic dragon as the Avestan Aži Dahāka; the literary genius of Ferdowsi was to fuse the two.
