Starting with Negal's (or Nergal) historical myth, he is said to be a god of war and pestilence. He presides over the Netherworld and is the head of a special pantheon assigned to the government of the dead. He is often depicted with the top half of a man and bottom half of a lion, like a centaur except the body of a lion instead of a horse. Negal is also mention in the bible as the deity of the city of Cuth. In demonology, he is seen as a deity of the desert, god of the underworld, and is at times identified with Satan. According to Collin de Plancy and Johann Weyer, Negal was said to be the chief of Hell's "secret police", and said to be "an honorary spy in the service of Beelzebub".
Negal first appears as a villian in Dr. Fate stories in More Fun Comics #67. In the story Negal is ruler of the Charnel Regions, which Dr. Fate refers to as the Dead World. Dr. Fate descends into the Charnel Regions to confront Negal about sending dead spirits into the living world. When Dr. Fate finds him, Negal is sitted in a throne-like chair holding a club and a skull. Dr. Fate defeats Negal by intimidation, making the skull that Negal is holding disappear and threatening to do the same to Negal.
Negal does not face Dr. Fate until the recent Count Down to Mystery mini-series in which Negal is looking for the next bearer of the Helm of Nabu. This happens to be the grandnephew of Kent Nelson, the original Dr. Fate, who is named after his predecessor. In the story, Negal is depicted like much more of a demon and ruler of the underworld. He is a Lord of Chaos, but is also referred to as the lord of pestilence. In the story he also refers to his initial meeting with the original Dr. Fate when the doctor descended in to the Charn Regions and threatened to destroy Negal.
Neither of the DC depictions of Negal are similar to that of the ancient Babylonians who (as stated before) depicted him as a half human-half lion being. In the early DC depiction, Negal is a giant with pale blue skin and not much muscle mass. In the later DC depiction, Negal has the look of a powerful warrior-king with dark blue skin. Much of the original Negal mythos is kept in tact by DC comics which makes Dr. Fate that much more of an interesting character and that much more fun to read.
Ancient Babylonian depiction of Negal (Nergal)
DC Comics' depiction of Negal