In Zoroastrian literature, Dahag, also known as Azi Dahaka, is one of the Azi (serpents, or dragons) that hold a reign of terror over the world.
He is a snake-like monster with three heads, three mouths, and six eyes, who apparently lived in an impassable fortress somewhere in Mesopotamia, where he regularly asked the more powerful divinities of water and wind for help with his hobbies, namely, depopulating the earth of humans, stealing cattle, sorcery, and collecting all the possible sins in the world (his name means “having ten sins”).
He was created by the “father of lies”, Angra Mainyu, who created him to counter Truth. With his paternal influence, and at least in part due to the traits he inherited from his mother, Odag.
He managed to become one of the most influential and feared evil forces in Zoroastrianism. He ruled the world with sorcery and the help of daevas (demons) for 1000 years, following the death of the good King Jam.
His reign was abruptly brought to an end when he was nearly beaten to death and imprisoned in a magic mountain by his nine year old successor, Fredon. He was not merely killed because he was filled with venomous creatures that would be let loose if Fredon were to kill him. Near the end of time/ judgment, he will be let loose, but will eventually be killed in the fiery river Ayohsust.