Call of Duty Black Ops 3 - Silverback in Black Achievement/Trophy Guide
In the safe house, inside your room, access the Data Vault computer. Navigate the cursor to the top left corner icon on the screen. After selecting it, a cinematic will start and you will be put into Dead Ops Arcade 2. Once the cinematic ends, you can begin playing this hidden mode, and you will also unlock the Silverback in Black achievement/trophy.