Doom Online

Doom (stylized as DOOM or DooM in other media) is a 1993 first-person shooter developed and published 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, Ultimate Doom, was released in 1995 and sold at retail.

Gameplay:
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.

Story:
Doom is divided into three episodes: “Knee-Deep in the Dead”, “The Shores of Hell”, and “Inferno”. A fourth episode, “Thy Flesh Consumed”, was added in an expanded version of the game, The Ultimate Doom. The game itself contains very few plot elements, with the minimal story instead given in the instruction manual and short text segues between episodes.

More info on Wikipedia.

Doom manual.

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