Asag ("demon that causes sickness") was a terrible chtonian demon from Sumerian legend responsible for draught and plague.
In the Sumerian mythological poem Lugale, Asag is described as being so hideous that his presence makes fish boil alive in the rivers.
He was said to be accompanied into battle by an army of rock demon offspring - born of his union with the mountains themselves. He was defeated by the god Ninurta.
Asag was often associated with the serpent or dragon mythological archetype by historians.  References
Jeremy Black and Anthony Green: Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary