Descent is an FPS/simulator developed by Parallax Software and published by Interplay, released in 1994 in Europe and in 1995 in the US.
The year is 2169. A computer virus of unknown origin has infected all the robots used in off-world mineral exploration, and turned them hostile. You are a mercenary hired by the company to go all over the solar system, rescue the workers, and destroy the robots.
Descent is a "6DOF" game, meaning you have six degrees (more properly six axes) of freedom. That is, you can move in any direction, turning and twisting without limits. It was one of the first fully 3D shooters (or almost: äll enemies were built with polygons, but items and hostages were prerendered sprites).
The FreeSpace series is an official spin-off to Descent.
The 2018 game Overload is a spiritual sequel to Descent, as it has very similar gameplay and was made by members of the original team.