Hunters in the Dark (Halo #16)

It is 2555, more than two years after the Master Chief went missing-in-action following a decisive conflict on the massive, extragalactic Forerunner construct known as the Ark as part of the final chapter in humanity's bloody thirty-year struggle against the overwhelming forces of the Covenant. When a startling scientific discovery is made,a hastily formed coalition of humans and Elites embark on a covert mission back to the Ark to save the entire galaxy.