Eternal is a free scenario for Aleph One, the multi-platform, open-source, first-person-shooter engine derived from Bungie's Marathon engine. It continues the story of the Marathon trilogy, and is a nearly total conversion, featuring:
Marathon was a landmark first-person shooter first released for the Macintosh in 1994, which introduced many new features and concepts to the genre including vertical aiming and mouselook; dual-weilded and dual-function weapons; versatile multiplayer modes such as King of the Hill, Kill the Man with the Ball, and cooperative play; friendly NPCs; and a deep and intricate narrative. The sequel, Marathon 2: Durandal, was released in 1995, improving on the engine technologies and greatly expanding the scope of the series. In 1996, Marathon 2 was ported to Windows 95, and the Marathon Infinity package was released, including a new scenario using a modified Marathon 2 engine, and most importantly, the tools used to build it, Forge and Anvil.
In the year 2000, Bungie released the source code to the Marathon 2 engine, and the Marathon Open Source project began, resulting in the new Marathon engine called Aleph One. Finally, in 2005, Bungie authorized free redistribution of the entire Marathon trilogy and all related files. This means that the entire trilogy can now be legally obtained for free and played on nearly any computer.