Before attempting to unlock quite possibly the most elaborate easter egg ever, make sure you satisfy the following:
Have 4 players. Everyone involved must have the trophies "Ensemble Cast" and "Time Travel Will Tell" from the previous Zombie maps, Call of the Dead and Shangri-La, respectively. Three Mystery Box weapons Wave Gun, Quantum Entanglement Device (QED), and Gersch Device. Excavator “Pi” to reach tunnel 6.
First, activate the power. Once you do, locate the four computer terminals that sit in the outside starting area. Each computer terminal lights up a specific color, and you essentially must play “Simon Says.” When you successfully input a round of sequences, all four terminals light up green, signaling the next step.
Head into the lab area with all the servers and use a hacker to hack one of four red buttons along the wall. Afterwards, some terminals light up green in the adjacent room. Have one of you hack all four of these green-lit terminals, and at least one other person press the four red buttons within seconds of one another until they all remain lit.
The third step involves the Excavator breaching tunnel 6 (where the M16 is). It’s easier if you only open up the doors through tunnel 6 when you begin. Afterwards, get rid of the Excavator by hacking the proper terminal in the Receiving Area.
If everything has gone according to plan so far, you’ll notice a round stone floating near the M16. Knife it to make it float around. Keep knifing it to move it along. Note that if it seems to have “hid” itself, simply shoot it in the general area where it disappeared to make it re-appear. This floating stone eventually bumbles its way to the disc at the base of that strange pyramid.
A cylindrical tube emerges from the right side of the pyramid’s base. Apparently, zombies’ souls belong in this tube. For the tube to collect these souls, you’ll need to kill several zombies near it, and you’ll watch the tube slowly fill up. When the tube is completely filled with zombie souls, you’ll next need to flip the switch on the wall to the right of the tube. Congratulations on unlocking Cryogenic Slumber Party! From here, you need Richtofen’s Vrill Generator to proceed. This is gotten by completing the previous Easter egg trophies. Whoever is Richtofen will have the Vrill Generator at the bottom of his screen.
All players will have the Death Machine power-up for about 90 seconds, so head back to Area 51 (where you first started) to earn some major points on zombies, if any. Look to your right and toss grenades at the plates on the shelf. A friend with the Gersch Device needs to then throw a Gersch Device to where the plates have fallen to suck them up and move them to the teleporter. Quickly get on the teleporter to be transported back to the station.
You’ll now find the plates lying on the floor in front of the Quick Revive. Use the QED to shuttle them to the computer in the corner of the same room. Whoever is Richtofen must place the Vrill Generator in-between the plates to activate a cut-scene. Have Richtofen repeatedly press SQUARE at the computer keyboard until the voice-over ends and the color of the computer screen turns and stays green.
Take the glowing Vrill Generator and return to the pyramid. The Vrill Generator will insert itself in the engraving and cause more cylindrical tubes to rise from the corners. You’ll need to kill enough zombies to fill up every tube. Once all four tubes have been infused with souls, activate the switch again to drain them.
Richtofen must grab the revealed Vrill Generator to earn him perks that stick around permanently. Use the QED to transport the floating stone back outside. It ends up right by the “Simon Says” computer terminals. Suck up the floating stone with the Gersch Device.
The stone goes to some out-of-reach rockets, which launch toward Earth, after completing “Simon Says.” Once the rockets smash into the Earth, you earn "Big Bang Theory."