Demons

Demons are a race of malevolent beings native to the plain of Hell who are usually subservient to more powerful Demons known as Devils. Some of these Demons are able to enter the realm of Earth under the guises of Humans but with noticeably inhuman traits, such as: glowing red/slitted eyes, fangs, incredible strength and speed, and an overall chilling demeanor. Each Demon on Earth has a distinct Demonic Release, a physical form more menacing and powerful than before.

Hierarchy of Demons
As there are different layers of Hell, there are different types of Demons, some more powerful than others.

Oni
The seemingly less-potent Demons, Oni are barely considered Demons due to their trace-amounts of Demonic heritage left. Though it seems Fenrir is the only shown Oni who is capable of using Hellsight, a Demonic ability. All Oni appear to be humanoid with physical characteristics of animals, such as dog/cat ears, fangs, tails, and (in rare cases) whiskers. They also behave in the manner of animals at times.

Šulak
Sadow encounters this type of Demon manifested in his Qliphoth, when his Demonic self is explaining his vulnerability. It tests Sadow against these Demons while in pitch-black darkness and it uses methods similar to an Anglerfish, though they use their light to find prey, not to lure them. When close, it will atempt to use one of it's four arms to grab hold of it's victim and devour them.

Succubi/Incubi
Demons governing Lust and subservient to Asmodeus, Succubi and Incubi are female and male Demons who appear, for the most part, humanoid with small bat wings attached to their heads and larger ones on their backs. They use the lust of humans as energy to fuel their power. It seems they can age as humans do when in Earth as Naamah appears to be an immature Succubus.