Apocalypse is a non-canonical Nazi Zombies storyline created by Daniel Smith. Apocalypse's storyline follows an alternate campaign of Call of Duty: Black Ops. The main characters are from the campaign, however some characters are custom made.
Daniel Smith got the idea of creating an alternate storyline from Magma-Man, the administrator of the Nazi Zombies Plus Wiki, after reading his creation called "Lockdown". Before Smith created the Apocalypse storyline, he created two other non-canonical maps following the Ascension storyline or Modern Warfare 2 storyline. They were "Moon" and "Zakhaev International Airport".
As stated above, the storyline follows an alternate storyline of Call of Duty: Black Ops. The maps are based of maps from the campaign and multiplayer, but more utilites have been added (or removed) for the Nazi Zombies game mode.
A sequel to the Apocalypse storyline was created, titled Apocalypse II: Left Behind.