Marathon Infinity
Marathon Infinity expands the closed universe of the Marathon series by quite a bit. The solo/co-op campaign "Blood Tides of Lh'owon" is a 20-level scenario with new textures, weapons and aliens. In addition, the scenario sheds a surprising new light on the story's characters and the meaning of the events. After defeating the Pfhor and resurrecting the ancient remains of the S'pht, the player now faces a world where friends become enemies and not everything is as it seems...
Story
The story in the single-player version of Marathon Infinity, entitled "The Blood Tides of Lh'owon", is not explicitly told. The narrative begins as if large parts, if not all, of the events in Marathon 2 did not take place. At the end of Marathon 2, when the Pfhor's Trih Xeem or "early nova" is fired at the S'pht system's sun to detonate it, Durandal recounts an ancient S'pht legend in which a chaotic being called the W'rkncacnter - an eldritch abomination - was sealed inside the sun eons ago by the Jjaro - a highly advanced ethnicity from centuries past whose technology is the only remnant of their existence. In the story, the player jumps back and forth between alternate realities in surreal dream sequences, trying to prevent the W'rkncacnter from being freed from the dying sun of Lh'owon. These jumps are caused either by the technology left behind by the Jjaro or by the chaotic nature of the W'rkncacnter. The player begins as Durandal's ally, but is then almost immediately transported to a reality where Durandal did not save the player at the end of the first game, Marathon, and is instead controlled by the Pfhoren-tortured AI Tycho.