Hop into a match of Blackout and go to the Asylum to trigger one of the most recognizable songs associated with the Call of Duty franchise -- Lullaby of a Deadman.
To find the Easter egg, go upstairs and find the restroom (which you basically can't miss). Go to the leftmost toilet and interact with it three times. You will hear water flushing. Soon after, the speakers all over the Asylum will start playing the Lullaby of a Deadman.
This is technically a returning Easter egg from the World at War, which a lot of fans of Call of Duty will know for sure. But it's great to see it making a comeback.