An asag (asakku) is a type of Sumerian Demon that attacks humans and kills them with head and fever diseases. Asag also is the proper name of a monstrous Demon in the Sumerian poem Lugale. Asag is the offspring of An, the sky god, and Ki, the earth goddess. He is hideous and has the power to make fish boil in their rivers. His allies are the stones of mountains.
The poem relates how Asag battles the hero god Ninurta and is defeated. This allows Ninurta to organize the world and use stones to construct the mountains so that streams and lakes flow into the Tigris and Euphrates Rivers, thereby aiding irrigation for agriculture.