Demons are a race of entities that inhabit Malbolge, serving under the goddess Ispa and her children. They are the antithesis of angels, but mirror the same social structure; being broken into seven groups based on seven major sins and led by a demon lord or arch devil. Though unlike angels, demons are no so concerned with serving the gods, usually they are left to their own devices and run their society as they wish in Malbolge. Some demons are employed in administering punishment to the souls toiling in Malbolge, however.

The appearances of demons vary based on sin. Typically rather hideous and fearful, they have traits of animals on humanoid bodies. Most have bat-like wings on their backs. When dealing with humans or Ispa, they often prefer to conceal their appearance with glamers to appear more human, though some traits (such as horns and wings) are still apparent.

Unlike most angels, demons are fertile and can reproduce amongst themselves and other beings. Ispa, Craeseth, and Sai'ras have all had children with the demon lords. Demon females may become pregnant by humans or other mortals through extra-planar sex. Children born of such a union are raised as demons and typically fall easily into the ranks of their mother’s kind. For a male demon to impregnate a human woman takes far more effort than a demon is typically willing to take just for a fling, unless he has ulterior motives, which would require transubstantiating on the material plane.


Demons are grouped into clans by the seven "cardinal" or "deadly" sins Wrath, Sloth, Pride, Lust, Envy, Greed, and Gluttony. Each clan has a specific beastly appearance.


Demons of Wrath have bear-like features. Their head is bear-like, but with some vaguely human features, especially about the eyes and skull structure. Massive curling horns wrap backwards from the top of their head and point forward at the jawline. Their arms and legs are massive and covered with black bear fur, ending with large paw-like hands with huge claws on each digit. Their torso is human-like, large, and thick with body hair—but not furred. Their wings are bat-like, heavily furred along the back of the arm. They are led by the demon lord Ohadibael.


Demons of Sloth have lion-like features. They have a lion head with vaguely man-like features about the eyes and skull structure, but still mostly feline, and with a thick, dark mane. Their arms and legs are a mix between man and lion, covered in dark, dingy dappled lion, thickly muscled and the legs digitigrade with fearsome retractable claws on the paw-like hands and feet. They have a lion’s tail, along with bat-like wings. Their torso is human. Demons of Sloth are led by the demon lord Sabatiel.


Demons of Pride have eagle-like features. They have the heads of eagles with fearsome hooked beaks, plumed in dark feathers. They have the torsos of humans, while their arms become wing-like with dark feathers pluming them from the shoulders down, distinctly along the back of the arm, which ends in talon-like hands. Their legs are bird-like, digitigrade, and ending in large black talons, the thighs plumed in feathers. Upon their backs are dark black plumed wings, as well as an eagle’s fanned tail. They are led by the demon lord Harsitiel.


Demons of Lust, also called succubae and incubi (succubus and incubus in the singular), have goat-like features, predominantly from the waist down, while their upper body is quite humanoid. They are known to have beautiful faces, but when appearing to humans, they often disguise their lower bodies which stand on digitigrade goat legs. On their heads are goat-like horns as well, and they have bat-like wings on their backs. Some Lust demons are also fully-functional hermaphrodites. They are led by the demon lord Arael.


Demons of Envy have jackal-like features. They have the head of a jackal with some vaguely human features, especially about the eyes and skull structure. They have lean, whip-like bodies with digitigrade jackal legs and short-haired black fur up their arms, which otherwise appear mostly human aside from claw-like nails on their fingers. They also have a jackal’s tail and bat-like wings. They are led by the demon lord Memurael.


Demons of Greed have raven-like features and are bird-like much in the way of Demons of Pride, only with the head of a raven rather than an eagle, and with more delicate talons. They also have plumed black wings. The feathers of both differ; both are dark, but eagle feathers are matte brownish-black while the raven feathers are glossy jet black. Greed demons are also leaner. They are led by the demon lord Asuderon.


Demons of Gluttony have boar-like features. Their facial structure is a mix of boar and human features, with boar ears and snout with fearsome tusks. They also have thick horns that arch back from the brow and then point forward. They are more portly than other demons, suiting their sin, and sit upon digitigrade boar legs. The rest of the body is human-like with bat-like wings. They are led by the demon lord Nazularuet.