From the Wikipedia entry on the son of Ahriman, Zahak:
"Zahhāk's tyranny over the world lasts for centuries. One night, Zahhāk dreams of three warriors attacking him. The youngest warrior knocks Zahhāk down with his mace, ties him up, and drags him off toward Mount Damāvand as a large crowd follows. Zahhāk wakes and shouts so loudly that the pillars of the palace shake. Following Arnavāz's counsel, Zahhāk summons wise men and scholars to interpret his dream. His hesitant counsellors remain silent until the most fearless of the men reports that the dream is a vision of the end of Zahhāk's reign at the hands of Fereydun, the young man with the mace. Zahhāk is thrilled to learn the identity of his enemy, and orders his agents to search the entire country for Fereydun and capture him. The agents learn that Fereydun is a boy being nourished on the milk of the marvelous cow Barmāyeh. The spies trace Barmāyeh to the highland meadows where it grazes, but Fereydun and his mother have already fled before them. The agents kill the cow, but are forced to return to Zahhāk with their mission unfulfilled."
