Aleph One (formerly known as the Marathon Open Source Project) is a free and open-source first-person shooter engine based on the source code of Bungie Studios' Marathon 2: Durandal. The project commenced in early 2000, when Bungie released the code shortly before being acquired by Microsoft and spurred the fan community to further develop it. Since that time, Aleph One has become a successful project in terms of development and community support. Most modern-day and die-hard Marathon enthusiasts use Aleph One due to its enhancements over the original games and the fact that it is playable with today's platforms. Its name is taken from the second infinite cardinal number in transfinite arithmetic and references Marathon Infinity, the final game in the Marathon Trilogy.
Marathon 2: Durandal was the first sequel in the Marathon series of science fiction first-person shooter computer games from Bungie Software. It was released on November 24, 1995. The game is mostly set on the fictional planet of Lh'owon, homeworld of the S'pht, and once again the player takes the role of a Security Officer from the Marathon.