Descent is a famous PC DOS shoot ’em up developed by Parallax Software and published by Interplay in 1995.
It is known as being the first ever First-Person Shooter to give full six degrees of freedom of movement. It was also apparently the first shooter to use “full 3D graphics”. Whatever that means.
In reality: Descent is an attempt to bring cockpit-based space shooters indoors. It’s basically: flying a spaceship inside corridors and tunnels, shooting stuff, and trying not to forget which way is up (not easy).
Still available to buy on GOG.com/Steam right now (at the time of writing) – along with the even better sequels – Descent is an interesting and playable relic from the post-Doom/pre-Quake DOS era.