Double Feature is essentially a Hardcore mode for Exo Zombies. After unlocking it in Descent, players can play it by finding a keypad located in spawn which will activate the mode. The keypad, however, requires all players in a session to interact with it, in order to activate Double Feature.
In this mode, a black and white filter is applied and players are given no heads-up display (other than the point counter), no teammate name indicator, and once a player goes down, they will die immediately instead of bleeding out. Thus, Exo Medic is disabled. Zombies also have more health than normal, and only reward points upon being killed. So weapons like the Blunderbuss should be favored in this situation. In addition, the story easter egg is also disabled and the Upgrade Station's price is reduced to 1500.