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:

• Fifty-two new levels, some revisiting familiar locations from the original Marathon trilogy.
• Over six hundred fifty new high-resolution textures, including a dozen original landscape textures, from a variety of human and alien environments.
• Eight all new weapons, many of which must be scavenged from your defeated enemies.
• Over a dozen tracks of original music, including many remixes of your favorite tracks from the original Marathon.
• And several new creatures and characters, alongside the complete cast from the original trilogy.

Picking up from the end of the Marathon trilogy...

...you find yourself suddenly ninety-four years in the future, in the year 2905. You are on the S'pht moon K'lia, hanging in orbit over a desolate and ruined Earth. Clearly all is not well with this future, and once again you are the last hope for mankind. The people of this time say that nobody really won in the war with the Pfhor; but now, thanks to recovered Jjaro technology, a plan has been devised to make things right. Paired with another sort of hybrid creature, the former Battleroid known as Hathor, you have been selected to venture back across time, one hundred and eleven years in the past to the U.E.S.C. Marathon. There, you and Hathor are to set in motion a plan that will alter the course of history and bring true victory to mankind. But things don't always go according to plan, and what begins as a mission to right history turns into an epic pursuit which spans not only the stars but also the centuries...

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 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.