Nergal comes to us from Mesopotamian mythology, the same that gives us Pazuzu, Tiamat and Lilith (after a fashion). He is an old god of the noon day sun, war, pestilence, disease and the planet Mars. It could be said that he even symbolizes the worst aspects of the god that would later go on to become Ares/Mars.
Like most old gods he was later demonized by Christian authors. He even appears in the Hebrew and Christian bibles as one of the few named demons (2 Kings, 17:30).
Christians would equate him with the Devil. Earlier Zoroastrianism would connect him with the Angra Mainyu, or their evil spirit.
In many of the myths/stories of Nergal he is the lord of underworld, but not the sole lord. It is a task he shares with his lover Ereshkigal, or as I like to call her, the world's first Goth chick. Depending on the story Nergal either rapes Ereshkigal, she tricks him into staying in the underworld or they have a loving relationship. All could be true.
In various games Nergal usually gets demoted. In AD&D 1st ed and D&D 3rd ed he gets only a mention as being one of the Rabble of Devilkin. Again this is unfortunate since it lacks a lot of what Nergal interesting. He gets a much better writeup in the 1st Ed Deities & Demigods and his girl friend is central to many adventures and games. I used her in my Army of Darkness/D&D mash up and again in my current 1st ed game with my kids.
Alignment: Neutral Evil
Armor Class: -3
Hit Points (Hit Dice): 110 hp (19 HD)
Attacks: Rod (x2)
Damage: 1d8/1d8 +save vs. death on critical hit
Special: +1 or better weapon to hit, command undead, immunity to fire, electricity and poison, regeneration (1 hp/round), see in darkness, magic resistance (55%), telepathy 150 ft.
Nergal is the Lord of the Underworld. While his origin is more closely aligned to the Baalseraph, he is closer in nature to a Demondand or Shedim. He is a demon that had been a god, but was killed and returned. He shares this quality with a number of other demon lords including Orcus.
He appears as a tall, older man with thick black hair and beard. He stands 7' tall, but can appear taller.He wears clothing fitting a king of his time (ancient Babylon) and carries a long rod that he uses in combat.
Like most kings he prefers to stay out of combat himself. Instead he can summon up to 4 dead enemies from a person's past to fight on his behalf.
He can also command any undead as if he were a 19th level evil cleric.
When angered his countenance changes to that of a lion with a long black mane and skin pocked as if with disease.
Nergal prefers to stay out mortal affairs except in times of war. Other powerful demon lords (typically Shedim and Baalseraph) employ him as a general or war marshall for their wars against each other.
While Nergal has lost much of his former power he still considers himself a god and not a demon.