"Demon" is the term applied to any species which originated from Hell. During their expeditions into Hell, the Union Aerospace Corporation (UAC) discovered a wide variety of species residing in Hell which were collectively dubbed Demons. While sapience and general intelligence varies between species of Demon, they are shown to be extremely aggressive against all forms of life, including each other on occasion, but ruled by a currently unidentified entity whose voice and testimonies are heard by the being known as the Doom Slayer. (Whether this is the Mastermind later fought by the Doom Slayer, an entirely different being or even the devil himself remains unclear).
Little is known about the history of the Demons or even their home-dimension, despite its common reference in various religions, although the Demons have been shown to be capable of accessing other dimensions beyond their own and forcefully integrating them into Hell. During an invasion of an alternative dimension known Argent D'Nur, the Demons were hopelessly outmatched by a group of extremely powerful warriors known as the Night Sentinels. However, they gained the advantage after bargaining with the bereaved leader of the Night Sentinels, offering him the Resurrection of his son in exchange for the Wraiths; the extremely powerful beings which ruled Argent D'Nur. As a result, the Demons claimed Argent D'Nur as an extension of Hell itself, enslaved the Wraiths to produce Argent energy for their own ends, and betrayed the Night Sentinels' leader by resurrecting his son, but in the form of a Demon. At some point in history after the fall of Argent D'Nur, Hell was invaded by an individual known to the Demons as the Doom Slayer; a humanoid being empowered by an angelic entity for the purpose of exterminating all Demons. As legions upon legions of Demons were slain by the Doom Slayer, a colossal Demon known as the Titan challenged him, only to be killed by his hand. Fuelled by unquenchable rage and bloodlust, the Doom Slayer was equipped afterwards with the praetor suit given to him by an unknown individual (The Demon recorded in the Slayer's testimony refused to identify them), enabling to heal his wounds through the absorbing of Argent energy from physical contact with Demons, typically while tearing them apart. Fearing the threat of mass extinction, the Demons lured the Doom Slayer into a trap, collapsing one of their blood temples on top of him and sealing him inside a sarcophagus, marking it with their rune identifying him to warn all Demons of what is trapped inside and that he must never be released. Upon establishing a research facility on the planet Mars, the UAC began to deploy expedition teams to Hell in order to harvest Argent energy, as the Human race had become dependent on it as a major energy source, and Demons to be experimented on for application to military use. However, one of the UAC's researchers, Olivia Pearce, was contacted by the Demons and, upon bargaining with them, converted almost the entirety of the staff of the UAC's Mars facility into a demonic cult, seeking to bring the Demons into their universe and sacrifice Humanity itself to them. Soon after the UAC's Demon specimens are released and the entire population of the facilities staff (apart from Samuel Hayden, CEO of the UAC, who had long since transferred his mind into a robot body to survive a terminal cancer diagnosis) are killed and reanimated by a wave of Argent energy from the UAC Hell portal, the Doom Slayer is released from his confinement. Extracted from Hell by the UAC and dubbed the Doom Marine, he was already present on Mars but inactive until the invasion began. Although the Doom Slayer/Marine was successful in killing the Demon known as the Spider Mastermind, who was apparently responsible for the invasion, along with countless Demons he encountered, future Demon activity is likely to be based on the planet Earth, along with the Doom Slayer's continued one-man war against Hell.
Possessed - Humans killed and reanimated by a wave of Argent energy, Possessed behave almost identically to Zombies, albeit more hideously deformed, but are easily killed. However, the process of creating Possessed seems to be selective, as UAC soldiers converted into Possessed retain their speed, strength and ability to use firearms.
Unwilling - Possessed which have been transferred to Hell, Unwilling are essentially a more skeletal version of Possessed and behave identically to them. Unwilling are used by the Demons as slave labour and cannon fodder.
Imp - The most common species of Demon, Imps are thin, red and roughly the size and shape of a Human. Although quite weak in terms of durability and damage output, Imps are capable of channelling Argent energy into their hands, producing a ball of fire which they can throw at their enemies. This fireball may also be charged for increased damage. Imps are also capable of scaling walls, allowing them to attack their foe from any vantage point.
Hell Razer - A Demon believed by the UAC to be host to a parasitic lifeform, the Hell Razer is armed with a chitinous growth in it's right arm which fires a continuous beam of argent energy. They also behave almost identically to Possessed soldiers which, along with their appearance and theorised origin, implies they may be a variant of Unwilling.
Pinky - A large feral Demon dubbed Pinky by the UAC, due to the other species' native to Hell having no name for it, Pinkies are extremely aggressive when faced with prey, recklessly charging at it while using the armour growing around its face as a battering ram to do so.
Hellknight - A huge brute bred purely for combat, the Hellknight, while lacking the agility of Imps or any means of ranged combat, is extremely durable and boasts incredible strength, capable of shrugging off most weapons fire, Hellknights will produce damaging shockwaves through smashing their fists into the ground, stunning their opponent.
Summoner - Believed to be an evolved form of Imp, Summoners are capable of levitation, short-range teleportation and firing blasts of argent energy capable of killing a Human in a single burst and possibly converting them into Possessed. Summoners are named for their most prominent ability; teleporting other Demons from Hell into the reality they are currently present in to serve as reinforcements.
Cacodemon - A large spherical Demon which is capable of flight and firing psionic blasts from it's mouth in order to immobilise prey for consumption. Cacodemons appear to be quite volatile, as puncturing their singular eye seems to cause them to explode.
Spectre - During their experimentation on Demons, the eye of a Cacodemon was transplanted into a Pinky, somehow causing it to become invisible. After the first Spectre escaped and returned to Hell, its descendants maintained this invisibility. Apart from this, they behave identically to Pinkies.
Revenant - Not quite a true Demon, Revenants are former Humans horrifically mutated and cybernetically modified by the UAC as part of a military experiment. Despite their origins, Revenants seem to be recognised as Demons by the denizens of Hell and are accepted into their ranks as such. Revenants primarily attack via shoulder-mounted rocket launchers and escape retaliation with their jetpacks.
Mancubus - A massively obese Demon outgrowing it's own natural armour, the Mancubus attacks its prey with a pair of organic flamethrowers in place of arms. Due to their diet of rotten meat, Mancubus innards have been made volatile and so puncturing the stomach can cause fatal rupturing. In their experiments, the UAC attempted to weaponise Mancubi by converting their flamethrowers into a sort of long-range plasma cannon. Unfortunately, the Cyber-Mancubus maintains its loyalty to Hell and continues to outgrow its UAC-issued armour, making them stronger than a natural Mancubus.
Lost Soul - Appearing to be a flying horned skull, Lost Souls display extreme aggression to anything they come across through charging at high speeds at their prey. Although Lost Souls are killed upon colliding with their target, UAC research has stated that they do so to take a 'host.'
Baron of Hell - boasting twice the strength and size of the Hellknight, the Baron of Hell is a massive horned beast capable of producing and throwing argent plasma at any enemies it is unable to tear apart with it's bare hands.
Cyberdemon - During their expeditions into Hell, the UAC recovered the remains of an extremely large Demon unlike any they had previously identified. As the Mars research facility became taken over by a Demon-worshipping cult, the giant cadaver was reanimated with cybernetic technology. The Cyberdemon was powered by an argent energy core in place of a heart and its left arm replaced with an energy cannon.
Hellguard - In their natural state, the Hellguard appear to be small leech-like creatures. However, they are capable of operating organic mech-like constructs in combat. Hellguard are extremely durable and are capable of projecting various energy blasts and shields from their maces.
Spiderdemon Mastermind - Believed to have been the mastermind behind the Demonic invasion of Mars, the true form of this Demon is unknown. After possessing a Human host, Olivia Pearce, the fusion of Human and Demon resulted in what appeared to be a giant brain-like creature housed within a spider-like weapons platform.