After you've played through the single-player game, have waited till the end of its credits, and played a round of it, you'll be able to do so as many times as you'd like. Simply visit the Mission menu, select "Nacht der Untoten," and you'll be on your way!
It first appeared in Call of Duty: World at War and became highly popular upon the game's release, resulting in the mode returning in Call of Duty: Black Ops, Call of Duty: Black Ops II, Call of Duty: Black Ops III, Call of Duty: Black Ops 4, Call of Duty: Black Ops Cold War, Call of Duty: Vanguard, and Call of Duty: ...
