Nergal, also known as Erra or Meslamtaea, was the god most frequently described as the ruler of the the Mesopotamian underworld, referred as either Kur or Irkalla, as well as a god of war and disease, sometimes described simply as a god of inflicted death. His main cult center was the city of Kutha, though he was popular all over ancient Mesopotamia. While many myths portray him as an unruly, chaotic deity with little respect for most of the other gods, he was frequently invoked in apotropaic rituals meant to ward off misfortune. He was also described as a punisher of evildoers. In at least one text he was referred to as the "sunset king" because of the belief that at night the sun traveled through the underworld. Some scholars therefore identified Nergal himself as a solar deity in the past. However, modern research generally only views the planet Mars as Nergal's astral aspect.
Nergal's main myths are the Epic of Erra, detailing his exploits in war, and Nergal and Ereshkigal, explaining the origin of his marriage to Ereshkigal, who was also a goddess of the underworld herself. It's possible the second myth was meant to reconcile two conflicting views on the underworld by making the central deities featured in them husband and wife. Unlike the ever popular Nergal, Ereshkigal only had a limited cult, and in fact one of the few known temples dedicated to her was located in Nergal's city, Kutha. Other goddesses, for example Inanna's vizier Ninshubur, were also sometimes viewed as Nergal's wives. His parents were Enlil and Ninil.
Nergal, like Ares and Mars, was sometimes associated with the planet Mars, which to the Mesopotamians was an ill omen associated with war and other calamities. His other symbols were the bull, the lion and a lion-headed mace.
Due to Nergal's fearsome nature he is known as a demon in popular culture, sometimes being identified with Satan. Colin DePlancy cites him as a spy in service of Beelzebub and chief of Hell's secret police.
Nergal is the second boss fought in the Ice Book bonus dungeon the Deep Hole. Upon defeat he joins the party.
Devil Survivor 2
"The god of the netherworld in Sumerian lore. Originally, he was the god of pestilence and war who lived in the heavens. While there, he refused to kneel to the messenger of Ereshkigal, goddess of the underworld. He was summoned there with 14 demons given to him by his father Ea. He intended to kill Ereshkigal with his sword, but she promised to become his wife and give him the netherworld's throne, so he spared her."