3D first-person shooter game

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.