In the Call of Duty: Black Ops III remaster of Nacht der Untoten, a radio message left by Doctor Monty discussing the backstory of the map can be found.
Radio 1[]
Doctor Monty: "Nacht der Untoten... That translates as 'Night of the Dead'. I learned that tonight. Been trying to scrub up on my German you see. Anyway, it happened in an old concrete building on an airfield somewhere...Christ, I don't know where. I remember it being really small, simple by today's standards. I think there was a fog. Not a lot of it, certainly not any coming into the building itself. You know what I just realised? You weren't even there! At least not at first. It was some other random, forgettable soldiers that had the misfortune to live through that horrible night, or nacht as it was actually known. There, I speak German now."