Doom is a 1993 first-person shooter developed by id Software for MS-DOS. Players assume the role of a space marine, popularly known as “Doomguy”, fighting his way through hordes of invading demons from Hell. The first episode, comprising nine levels, was distributed freely as shareware and played by an estimated 15–20 million people within two years; the full game, with two further episodes, was sold via mail order. An updated version with an additional episode and more difficult levels, Ultimate Doom, was released in 1995 and sold at retail.
Along with its predecessor Wolfenstein 3D, Doom helped define the FPS genre and inspired numerous similar games, known as “Doom clones”. It is one of the most significant games in video game history, and is frequently cited as one of the greatest games of all time. It pioneered online distribution and technologies including 3D graphics, networked multiplayer gaming, and support for customized modifications via packaged files (WADs). However, its graphic violence and hellish imagery also made it a subject of controversy.
Doom has been ported to numerous platforms. The Doom franchise continued with Doom II: Hell on Earth (1994) and expansion packs including Master Levels for Doom II (1995). The source code was released in 1997, inspiring further adaptations. Id returned to the franchise with Doom 3 (2004), a horror-focused retelling using the id Tech 4 engine, followed by a 2005 Doom film. A new series, returning to the fast-paced action of the originals, began with the 2016 reboot Doom.
Doom is a first-person shooter presented with early 3D graphics. The player controls an unnamed space marine—later termed the Doomguy—through a series of levels set in military bases on the moons of Mars and in Hell. To finish a level, the player must traverse through the area to reach a marked exit room. Levels are grouped together into named episodes, with the final level focusing on a boss fight with a particularly difficult enemy. While the levels are presented in a 3D perspective, the enemies and objects are instead 2D sprites presented from several set viewing angles, a technique sometimes referred to as 2.5D graphics. Levels are often labyrinthine, and a full screen automap is available which shows the areas explored to that point.
While traversing the levels, the player must fight a variety of enemies, including demons and possessed undead humans, while managing supplies of ammunition, health, and armor. Enemies often appear in large groups, and the game features five difficulty levels which increase the quantity and damage done by enemies, with enemies respawning upon death and moving faster than normal on the hardest difficulty setting. The monsters have very simple behavior, consisting of either moving toward their opponent, or attacking by throwing fireballs, biting, and clawing. They will fight each other if one monster is accidentally harmed by another, though most monsters are not harmed by other monsters of the same kind. Levels can also include pits of toxic waste, ceilings that lower and crush anything below them, and locked doors which require a keycard, skull-shaped key device, or a remote switch to be opened. The player can find weapons and ammunition placed in the levels or can collect them from dead enemies; weapons include a pistol, a chainsaw, a plasma rifle, and the BFG 9000, among others. The levels also feature power-ups such as items that give health or armor points, increase the player character's maximum ammunition or health, fill out the automap, give partial invisibility, or allow the player to survive in toxic waste. There are also items which apply time-limited effects such as invulnerability or a berserker status.
In addition to the main single-player game mode, Doom features two multiplayer modes playable over a local network: “cooperative”, in which two to four players team up to play through the main game, and “deathmatch”, in which two to four players play against each other. Online multiplayer was later made available a year after launch through the DWANGO service. Doom also contains cheat codes that allow the player to be invulnerable, obtain every weapon, be able to instantly kill every monster in a particular level, and several other abilities.