Marathon was initially developed as the second part of Pathways into Darkness, but during the development process this idea was abandoned as the game evolved on its own to become a futuristic shooter that was played by thousands of players during the 90s.
Furthermore, Marathon didn't settle for just one game, it ended up evolving into a world famous saga, that included two other titles: Marathon 2: Durandal and Marathon Infinity.
Unlike other games, Marathon used its own game engine that, with the release of the code suffered with the game, also ended up being adapted and modified, finally receiving the name of Alpha One.
The graphics of Marathon are rather bad and archaic if they are compared to those of the modern day games, but the game still offers great playability. Furthermore, despite being an action game Marathon placed special emphasis on the game's story, that was set in the future, which hadn't had any relevance in shooter games up until that moment.
Therefore, if you want to play this great classic game again, you only have to download Marathon.