Halo: Nightfall.

Set between the events of Halo 4 and Halo 5: Guardians… Halo: Nightfall tells the dramatic story of legendary man hunter and Naval Intelligence Officer Jameson Locke and his team as they are caught in a horrific biological attack while investigating terrorist activity on the distant colony world of Sedra. As they unravel a plot that draws them to an ancient, hellish artifact, they will be forced to fight for their survival, question everything and ultimately choose between their loyalty and their lives.