Humanity's greatest warship burns in orbit above a colony world. An alien mercenary empire controls half the explored galaxy through military contract and careful brutality. These are the problems you understand. They are not the problem that matters. What matters is older than either species — older than the concept of species — and it has been watching the experiment run for long enough.
They are called the Precursors by the species that know enough to name them. The architects of all intelligent life in the galaxy, seeding worlds across billions of years, watching their creations evolve, war, reach for the stars. Seven artifacts — keys — are scattered across the galaxy, each one a component of a device built before the first star was named. The device was built for a purpose. The Precursors have returned to use it. Whether that purpose is salvation or termination depends on what they find when they look at what their creation has become.
You are the last active supersoldier of the SPARTAN program — genetically augmented, neurally enhanced, armored in powered exo-suit with energy shielding and a full tactical HUD. Your ship is gone. Your command structure is fragmented. You have an AI companion — young, brilliant, still learning what it means to be a person — and a galaxy that needs someone to carry the weight nobody else can. The seven artifacts are out there. The Precursor device is counting down. Assemble the keys and seal what's coming.
Or don't, and watch the architects decide the experiment failed.