I'm surprised I hadn't heard of this one before! A few developers have been building Nazi Zombies: Portable, a Call of Duty: Zombies demake powered by various enhanced forks of the Quake engine.
Anyone who has played Call of Duty: World at War is familiar with the end credits, in which players watch a sprinting zombie burst into frame when the scorched words “Nazi Zombies” appear.
Treyarch confirms exactly when the 'main quest' Easter Eggs unlock for the Call of Duty: Black Ops 6 Zombies maps.