For awhile, Doom was perhaps the most realistic FPS kicking around, as laughable as that sounds. It was, though, and it's only natural that authors would try to manipulate the core aspects of Doom's gameplay in order to facilitate an experience even closer to conceptions of what a realistic first person shooter experience would be. Two wild and crazy guys - "Ziggy" and "Sidearm Joe" - made a series of levels called Hostage Rescue that tried to emulate this experience, but their efforts came to a head in 1998 with the release of Twilight Warrior, published as Black Shadow Software with the aid of some other authors, some now quite famous in the Doom community. It's a ten-map episode for Doom II, originally designed to be played in what is now an ancient version of the Legacy engine. Nowadays, any modern port that supports mouselook, jumping and DeHackEd should be able to run it.
