Requiem databases recover old files of an incident in the United States Department of Defense, where a major security breach was located within the depths of the Pentagon.  Leading to a much larger picture, Requiem is beginning to piece together the past as it realizes just how serious this issue is.
Based on Call of Duty's Zombie horde mode, originating in 2008's "World at War" and has since led to 10 variations of the mode across 16 years, including 3 non-Treyarch developed modes and 2 more additions in non-Treyarch titles.