Thirty-two years ago, contact with the outer colony Harvest was lost. A battlegroup sent to investigate was almost completely destroyed; only one badly damaged ship returned to Reach. Its crew told of an unseemingly unstoppable alien warship that had efortlessly annihilated their forces.

This was humankind's first encounter with a group of aliens they eventually came to know as the Covenant, a collective of alien races united in their fanatical religious devotion. Covenant religious elders declared humanity an affront to the gods, and the Covenant warrior caste waged a holy war upon humanity with gruesome diligence....