Halo: Combat Evolved

In a desperate attempt to keep a conquering alien race called the Covenant from finding Earth, you--a cybernteically enhanced soldier representing the peak of human military evolution--and a ragtag group of Marines flee to a distant uncharted system where you unexpectedly stumble across Halo, an enormous alien artifact in orbit between a gas giant and its moon. Your objective: Uncover Halo's horrible secret, and destroy mankind's nemesis - the Covenant.