Former human monsters are zombies possessed by the evil forces of Hell, and are, across all of the Doom series, the most common opponents of the player when considered as a group. Inside the Doom source code and in the Doom Bible they are referred to as possessed humans, and their sprite prefix in the classic series contains an abbreviation for "possessed" (ie., POSS, SPOS, CPOS). Because of their human origin, they attack with their bare hands or by using hand-held firearms instead of using innate attacks or cybernetic implants as other monsters do, and are generally easier to eliminate than most of their fully demonic counterparts. Because of their relatively low hit points, they are often gibbed when struck by more powerful opponents.
In most of the games, armed former humans drop weapons or ammunition when killed; this sets them apart from the purely hellish monsters. A dropped item remains even if the zombie respawns or is resurrected by an arch-vile; if killed again, the zombie will drop yet another item. Being humans that somehow became possessed by demonic forces, they are not generated by the final boss of Doom II.
Former human types
The military forces on the moons of Phobos and Deimos, and on Earth in Doom II and its sequels, were possessed en masse when the invasions of those areas began. Zombification was known of before the invasions, however, and was noted during the earliest teleportation experiments. In the game's instruction manual, test subjects were said to become violent, babble vulgarities, and eventually suffer from "random full-body explosion." The latter mechanic does not actually occur in-game.
In Doom 3, possessed humans belong to one of three main classes: ordinary unarmed basic zombies, the firearm-bearing Z-Secs, and the elite commandos. The first two are created through direct possession by evil spirits, which appear as translucent flying skulls at the start of Hell's invasion. Commandos on the other hand were created directly by Dr. Malcolm Betruger, through use of demonic magic. As such they display a higher level of intelligence and brutality, and share more in common with demons than with the lesser possessed humans.
Simply visiting Hell could result in a gradual form of zombification similar to that described in Doom's instruction manual. Instances of this increased as Delta Labs experimentation with teleportation increased in frequency, and the UAC's doctors were unable to explain the process.
In the 2016 Doom, the possessed are a class of monsters who were created from the bodies of UAC employees who were subjected to Lazarus waves and Hell waves. In the hierarchy of Hell, possessed are the lowest level and are treated as slaves, forced to perform labor such as constructing temples and tending to gore nests. Possessed who are taken to Hell eventually further transform into unwilling.
- In the Doom Bible, a sketchy plan of the first level of the game, called "Level one: Secondary hangar", marks the locations for "dehumans"; perhaps using the "de-" from demon to make a reference to the Black Sabbath album from (June) 1992, Dehumanizer, especially considering Tom Hall has demonstrated a penchant for inserting popular references into his game designs.