The Black Ops Cold War zombies keycard is a special item that can be used to obtain Wonder Weapons on the map Die Maschine.
The golden keycard drops once a player has successfully defeated a radioactive Megaton Blaster after it has already split into two parts. Once picked up, it stays in the player's inventory for the rest of the game.
Black Ops Cold War Zombies Keycard: Die Maschine Easter Egg Guide
The first thing players need to do upon starting the game is turn on the power, and build the Pack-a-Punch machine.
After the player has obtained the keycard, they should go down into the weapons lab, look for the weapons locker with a blinking red light, and insert the keycard to receive the D.I.E. Remote Control.
The D.I.E. Remote Control is used to activate a D.I.E. that is locked behind a door inside the graffitied Nacht der Untoten part of the map. Activate this D.I.E. to use as a trap to kill zombies chasing you.
Once the D.I.E. has absorbed enough souls, the player will be able to discharge it with the D.I.E. Remote Control, causing it to blow open the door.
Inside the small room will be a skeleton holding the base rarity of the D.I.E. Maschine Wonder Weapon, available to be picked up for free.
For more on Black Ops Cold War, feel free to check out our guide on how to complete the Nuketown '84 Easter Egg. Also, here's how to unlock the three blueprints on Nuketown '84's Zombies Onslaught mode exclusive to PlayStation players at the moment.