Asag is a black evil demon spirit  from the underworld that is mentioned in Sumerian mythology; he is said to cause disease and sickness.  In Sumerian mythological poem Lugale, Asag is described as being "Huge, rounded, three-legged, three-armed animal with no neck and a few eyes covering the whole mass, he has a dim, solidified skin that is practically indestructible.  Asag is said to be one of the top ten demons to avoid if at all possible in regards to conjuration.