The Mesopotamian god of plague, war, and the underworld was Nergal.
Nergal was a major deity of inflicted death, combining the destructive forces of warfare, disease, and the realm of the dead. He was particularly associated with the Mesopotamian underworld, known in Sumerian traditions as Kur, and in later mythology became the husband of Ereshkigal, its powerful queen.
Nergal’s chief cult center was Kutha in northern Babylonia, where his temple E-Meslam stood. His symbols included a lion-headed mace and a sword, while lions and bulls were among the animals connected with him. He was also associated with Mars, a planet whose reddish appearance suited his violent and dangerous character.
The commonest confusion is Erra. Erra was originally a distinct Akkadian god of plague and destruction, but the two deities were syncretized early and their names can function as near-synonyms in literary texts. Namtar was instead Ereshkigal’s attendant and a personification of fate or disease, while Pazuzu was a wind demon who could protect against Lamashtu.