Marathon is an FPS by Bungie Software, released in December 1994 for the Apple Macintosh computer line.
The year is 2794. You are a security officer at the UESC Marathon, a colony ship build from the Martian moon of Deimos, on the way to the Tau Ceti IV colony. The ship is attacked by aliens called Pfhor, and it is revealed that one of the ship's artificial intelligences has gone rampant and is assisting the Pfhor for its own goals.
Marathon was originally released for the Macintosh. It was subsequently released for the failed Apple/Bandai Pippin console as a 2-in-1 pack with Durandal titled Super Marathon. The series was re-released in 1997 as Marathon Trilogy Box Set.
After Bungie was acquired by Microsoft, they made the Marathon series open-source and freely distributable, which led to fan-made ports and technical improvements.