Twenty years ago this week, a studio called id Software released a first-person shooter game featuring a space marine who must shoot his way through a demon-infested Mars base. The game was called Doom, and everything from its blocky 8-bit graphics to its frenzied premise might seem ridiculous to modern viewers, but the games impact on modern first-person shooters — and modern video games in general — cannot be overstated. Here are 10 games that just wouldnt exist if Doom hadnt paved the way. 1. Marathon (1994) Doom is considered the first game to employ networked multiplayer, but Marathon, released a year later by developer Bungie Software for the Macintosh computer, was the first to do it well. Marathon incorporated many of the features pioneered in Doom, such as cutting-edge (at the time) graphics, while adding some new features of its own, such as letting players look up or down by moving the mouse. After cutting its teeth on the Marathon trilogy, Bungie would later become famous for its Halo video games. 2. System Shock (1994) Developed by Looking Glass Studios for Microsoft DOS and the Macintosh operating system, System Shock is a first-person shooter set aboard a space station in which players have to contend with a malevolent A.I. called SHODAN. While developing the sequel, System Shock 2, an offshoot of Looking Glass Studios became Irrational Games, the company that went on to release BioShock and BioShock Infinite. 3. Mod culture This one isnt a game, but fan-made and fan-distributed mods, or modifications, are an important trend in the gaming world that owe a huge debt to Doom. The lead programmer of Doom, John Carmack, designed the game in such a way that it was easy to edit or swap out entire files or sections of code, letting people create new levels, characters and environments. In 1997 Carmack made the games entire source code public under a GNU (General Public License), making it even easier for amateur and aspiring game designers to modify the game and create entirely new games. 4. Star Wars: Dark Forces (1995) With all the fan-made Star Wars-themed Doom mods popping up in the 90s, it was only a matter of time before LucasArts made an official Star Wars first-person shooter. That game was Star Wars: Dark Forces, and it marks the first appearance of mercenary-turned-Jedi Knight Kyle Katarn, who has since appeared in a number of Star Wars video games and novels. 5. Duke Nukem 3D (1996) First released for the MS-DOS by developer 3D Realms, Duke Nukem 3D was to Doom what todays Saints Row is to Grand Theft Auto: a spoof of the violence and machismo of the increasingly popular first-person shooter genre. Today, Duke Nukem 3D is remembered as one of the games that helped popularize the first-person shooter genre. MORE: Is Half-Life 3 the New Duke Nukem Forever? 6. Quake (1996) After making Doom, id Software released Quake, a first-person shooter that took everything fans loved about Doom and made it better. The games underlying framework, called the Quake Engine, featured improved 3D graphics and a more robust physics engine. The original Team Fortress released in 1999 by Valve Corporation is just one of the games made using the Quake Engine. 7. The Elder Scrolls II: Daggerfall (1996) After the first Elder Scrolls game, Arena, was released to a mixed reception in 1994, small studio Bethesda Softworks returned to the drawing board with Daggerfall, a game that combined role-playing game elements with the increasingly popular first-person genre. MORE: PS4 vs Xbox One: Which Console Wins? Like Doom, Daggerfall has all the creepy first-person monster killing a gamer could want, but Daggerfall also contextualizes its action in the sprawling fantasy world of Tamriel, which would itself inspire the open-world games of the late 1990s and 2000s. 8. Turok: Dinosaur Hunter (1997) Pretty much any non-military first-person shooter with a totally wackadoodle plot is an homage to the Martian demon-killing space marines of Doom. Of these games, one of the most well-known and long-lived is Turok: Dinosaur Hunter for the Nintendo 64 and Microsoft Windows, whose main appeal involves playing as a dinosaur-killing space marine. 9. Half-Life (1998) Crazy as it may sound, the insane plot of Doom helped inspire what may consider one of the best examples of story in video games to date. Half-Life by Valve Corporation is a first-person shooter made using a heavily modified version of the game engine from Quake, Dooms younger, shinier sibling. Combining first-person perspective with puzzles and a seamless narrative that used ambient dialogue instead of cutscenes, Half-Life proved that first-person shooters can be smart as well as fun. 10. Halo: Combat Evolved (2001) The legacy of Doom is evident in Bungies Halo: Combat Evolved for the original Xbox, from its fast-paced combat to its space marine protagonist. But Halo took the Doom model and changed it in a key way that became a turning point for the first-person genre. In Doom and its offshoots, players had a set amount of health points, meaning any given part of a level could be either very hard or very easy depending on how much health they had left. By giving players regenerating health instead, Halo: Combat Evolved was able to create consistent difficulty across its levels, which made the genre more accessible and opened up whole new avenues for level design and pacing. Email [email protected] or follow her @JillScharr and Google+ . 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