Call of Duty: Black Ops 3 contains a hidden top-down twin-stick shooter called Dead Ops Arcade 2: Cyber's Avengening.
As captured by Kotaku, this mini-game is accessed from the a terminal in the game's hub. Interact with the icon on the dashboard's top left icon and you'll launch into Cyber's Avengening.
This mini-game follows the story of a soldier whose beloved rooster has been kidnapped by a cybernetic ape whose own fowl companion recently passed on. Making matters worse, the ape descended into a haunted giant skull, leaving you to contend with waves of incoming zombies as you trek through this undead island.
There are plenty of power-ups for more powerful weapons (like a flamethrower), branching paths, and one item even allows you to play a stage in first-person.
Take a gander at Dead Ops Arcade 2: Cyber's Avengening in Kotaku's video below: