Doom is a 1993 science fiction horror-themed first-person shooter video game by id Software. It is considered one of the most significant and influential titles in the video game industry, for having ushered in the popularity of the first-person shooter genre. The original game was divided into three nine-level episodes and was distributed via shareware and mail order. The Ultimate Doom, an updated release of the original game featuring a fourth episode, was released in 1995 and sold at retail. In Doom, players assume the role of an unnamed space marine, who became popularly known as “Doomguy”, fighting his way through hordes of invading demons from Hell.With one third of the game, nine levels, distributed as shareware, Doom was played by an estimated 15–20 million peoplewithin two years of its release, popularizing the mode of gameplay and spawning a gaming subculture. In addition to popularizing the FPS genre, it pioneered immersive 3D graphics, networked multiplayer gaming, and support for customized additions and modifications via packaged files in a data archive known as “WADs”.