Nergal (ネルガル, Nerugaru)? is a demon in the series.
Babylonian lord of the underworld who was also a solar deity (though only representing a certain solar phase) identified with Shamash. He is also portrayed in myths and hymns as a god of war and pestilence. Being a deity associated with the desert, fire and the underworld, as well as a pagan god of a rival religion, made him a prime target for Christian demonization. Thus he is known as a demon in conventional culture, sometimes 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."