Nergal is a black mutant monster from the Center of The Earth who has snake like tentacles growing form his back. These tentacles can electrify people and act as extra arms. Nergal can also alter his shape and form, into almost anything he desires at will; his son inherits this same capability. He wears a red and black business suit. He is named after the Babylonian fire god.
Only in the episode "Love Is EVOL Spelled Backwards" and the video game, Nergal actually had friends (they're also minions). They are called "Nergalings," and are his only friends.
Nergal is often seeking friendship, but is typically unsuccessful. Nergal later married Billy's Aunt Sis. After a string of unsuccessful attempts to woo Sis, it was Billy's backfired attempt to sabotage Nergal's and Sis' date that won Sis over, thus making Nergal Billy's maternal uncle by marriage. Nergal has one son, named "Nergal Jr."
Nergal's character is most likely based to at least some degree off of the Sumerian God of the same name. Nergal was the Sumerian ruler of the Underworld, and god of the dark aspects of the Sun [which could take away life, especially since Sumeria was situated in what is today the Middle East, which is a desert region].
Nergal can shapeshift into any shape and form he desires, possess the bodies of humans, electrify enemies by ensnaring them in his snake-like tentacles, and teleport in a flash of pure white light.
Some have noticed similarites between Slenderman and Nergal.