The chaotic fiends inhabiting the Abyss. They are ferocity personified and will attack any creature just for the sheer fun of it—even other demons.
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Tanar'ri
The tanar'ri are a race of numerous demons originally created by the obyriths as slaves. The tanar'ri eventually revolted against the obyriths, killing most of them, and taking over as the dominant race of demons in the Abyss. Most known demon lords are tanar'ri, with the exceptions being listed above.
The tanar'ri are essentially "classic" demons; reflections of cruelty, evil and sin. Although there are several exceptions, they usually have a basic humanoid form.
There are many known types of tanar'ri, including: Adaru, alkilith, alu-fiend, anzu, armanite, arrow demon, babau, balor, bar-lgura, bulezau, cambion, cerebrilith, chasme, dretch, gadacro, glabrezu, goristro, hezrou, jarilith, jovoc, kastighur, kelvezu, klurichir, mane, marilith, maurezhi, molydeus, myrmyxicus, nabassu, nalfeshnee, orlath, palrethee, rutterkin, skurchur, solamith, sorrowsworn, succubus, turagathshnee, uridezu, vathugu, vrock, and yochlol.
Obyrith
The obyriths are so ancient that they predate mortal life, and even the gods. They rarely have a humanoid shape, and some say that just looking at an obyrith can drive a mortal insane. Their great age and apparent ability to instill insanity at a glance are strong hints at some relation with the Great Old Ones of other universes. A few types of remaining obyriths are draudnu, ekolids, laghathti, sybriexes, & uzollru. Some obyrith lords have evolved over time to take on more recognizable shapes, such as Pazuzu or Pale Night. Other known obyrith demon lords include Obox-ob, Dagon, the Queen of Chaos, the Malgoth, Ugudenk, Bechard, Vroth-Khun, Ubothar, and Cabiri (imprisoned in the Wells of Darkness). Asima and numerous other obyrith lords whose names were lost to time have been destroyed, either slain by each other, or slaughtered by the Tanar'ri.
Ekolid
Ekolids belong to the ancient race of demon called obyrith. Being obiryths, ekolids have monstrous forms, which can drive mad anyone who dares look at them. At first, they resemble the union of an ant, a scorpion and a spider, but their real fiendish shape can not be compared to any other creature. Ekolids are driven by the need to reproduce. During any battle, using their supernatural speed, they constantly try to implant egg in their enemies. Ekolids are rarely found in groups, except on Zionyn (663rd layer of the Abyss), where they live at the service of Obox-ob.
Armanite
Armanites compose the heavy cavalry of the army of The Abyss. They resemble fiendish centaurs covered by a full plate armor. They are fierce enemies in battle and their charges are deeply feared. On the layers of the Abyss they usually wander in groups guided by a chief called knecht or pathwarden. Because of their unusual discipline, they are often employed by powerful abyssal lords as mercenaries. Most of armanites come from the Plains of Gallenshu (377th layer of the Abyss); on this layer there are 24 cities of armanites, each ruled by an armanite called konsul.
Bulezau
Bulezaus are the core of the heavy infantry of the armies of The Abyss. Bulezaus have the shape of a big man with the head of a ram. Despite their appearance they are good warriors, particularly in melee combat. They aggregate in groups only if they have a powerful leader able to control them; whenever this leader passes away, it is almost certain that bulezaus will start fight each other until death.
Chasmes
Chasmes are merciless demons resembling great flies. They enjoy torturing and tormenting other beings, also demons, especially if they are sure of their impunity. Because of their cruelty, chasmes are often charged with torturing enemies and deserters. Sometimes chasmes can be found in group acting as patrols; they fly in the skies of The Abyss looking for intruders.
Loumara
The loumara are a relatively new demonic race, much younger than the tanar'ri. They are still centuries old, but this is still recent in a place like the Abyss, where time doesn't mean the same thing it does on the Material Plane. As a result, no loumara has become powerful enough to be recognized as a demon lord.
Loumara are usually immaterial or invisible demons that are more like ghosts or undead than demons. All loumara can possess living creatures.
Known types of loumara include:
A Dybbuk resembles a jellyfish with a simply sketched human face. Dybbuks are incorporeal creatures and so they do not have a specific physical form. Dybbuks can possess and control dead bodies, so they spend a lot of time looking for a 'perfect' host body to animate – one that died without violence or major injury, and preferably one that is handsome or beautiful. After taking the control of a 'perfect' body, dybbuks try to insert themselves into the society of the victim, plunging into depravity and hedonism until their possessed body is broken down and ruined. When a 'perfect' body is not available, a dybbuk will take a less desirable body and continue to search for a better host. Dybbuks are lonely creatures, and can be found in groups only where there are many corpses to possess, such as on a recent battlefield. They tend to avoid undead, because already-animated bodies are of no use to them.
A Guecubu resembles a mass of vapor, and is born from dreamstuff tainted by evil. Guecubus can possess and control the body of sleeping humanoids. Once they are in control, they 'ride' silently, letting their victim go about his or her business. At some point they take control and lash out, trying to spread death and murder – particularly on the victim's family and friends. Sometimes the guecubu will conceal its actions even from the victim, letting them believe that they are cursed or jinxed.
Guecubus believe that killings form some sort of pattern, and enough spilled blood will eventually reveal the meaning of creation in this pattern. They rarely form groups with other guecubus, preferring to remain hidden and anonymous. However, many of them can be found in the Dreaming Gulf (230th layer of the Abyss), where they are created spontaneously from the raw, churning chaos of dreams.