This Easter Egg can be performed either in solo or co-op!
Items and steps needed to complete the Easter Egg:
To the left of your starting position are two tombstones floating on a small island right outside of your area. These are tombstones 1 and 2. Moving farther down is a buyable barrier that opens a jump pad leading to Origins. In the opening between the couch and the wall, straight ahead are two more tombstones. These will be 3 and 4. You need to shoot the stones in the order that the characters have died in the previous maps: 2, 3, 1, 4. A red flame will appear from each stone if you have done this step correctly.
It's a good idea to go ahead and start activating the Corruption Engines now as you'll need to have them done after Step 2 and it will be easier to kill the Furies and Keepers.
There are four locations: Spawn, Der Eisendrache, Mob of the Dead, and Verrückt.
Click here for a guide to creating the Keeper Protector
After shooting the gravestones, a small, triangular rock will appear at one of the jump pads around the map, either at Mob, Shangri-La, or Verrückt.
Once you find the correct jump pad, you'll need to stand on the rock so the Keeper can begin its ritual. You won't be able to stand on it without setting off the jump pad, so use the pad to cross to another island, then use the pad there to go back and stand on the rock while the pad is cooling down. You only need to stand on the rock until the ritual begins as you'll need to stay in the area and protect the Keeper from zombies who will begin attacking it. The ritual lasts around 3 minutes.
After the ritual is complete, an audio reel will appear on the ground, so all you need to do is pick it up and take to the player in Nacht der Untoten, upstairs sitting on a wooden box.
Lil Arnies are needed for this step. You can obtain them from the Mystery Box.
By now, you should have the 4 Corruption Engines activated. Once complete, head upstairs in Nacht and find the control panel with the 4 green lights. Wait for the Apothicon to fly by and interact with the console as its lights flash to trap it. The barrier to the right will open and you can run over the Apothicon's mouth where it will rope you in.
While you're here, release the Pack-a-Punch if you haven't already as you'll need it for the next step. Once you're in the Apothicon, look straight ahead and up and you can see it inside of an organ. Drop it by shooting the 3 connections holding the organ in place.
In the Apothicon are 9 small holes that occasionally have a gas emit from them. Throw one Arnie into each of them. It can be difficult to get them in so be careful. After ever 3 Arnies, 3 Margwas will spawn in so you'll need to take care of them as well.
After the 9th Margwa is killed, the second audio reel will appear on the bridge which you'll take to the second player located on the Kino der Toten stage on the opposite of the teleporter.
Around the map will be 6 special rocks. Shoot each of these with an upgraded gun and a bone will appear from them. Using the Estoom-Oth (Upgraded Estulla Astoth), fire it at the bone to suck it up.
Rock and Bone Locations:
All the bones you've collected will now have appeared on the floor, upstairs in Nacht, across from the console. Shoot the Estoom-Oth to collect them and Sophia's body will replace them. Shoot the body to collect it and the third audio reel will appear which can then be taken to the player in Origins, sitting on a table at the top of the area.
After playing the reel, a transparent version of the Sophia robot, as seen in Gorod Krovi, will be in one of the openings in the roof. Going to each Engine, there will be two blue crystals spinning in the sky. Activate it and aim at them, eventually one of them will reflect the beam correctly and make a connection with Sophia at Nacht. Do this with all 4 Engines.
Return to Nacht after all 4 beams have connected and she will become solid, so follow her as she floats through the map to the Kino teleporter where she will sit above it and activate it.
Hop in the teleporter where you'll then be transported to Samantha's room. On her bed is the Kronorium, pick it up and you'll be teleported back.
Place the book on the podium next to the teleporter which will then shoot 4 orbs up and across to the projector room, placing 4 symbols on the floor.
If you have the spark, bring it with you to Samantha's room and you can place it above the chest to the left of where you spawn in, with the Mystery Box "?" drawn on it. Doing so will grant you access to Takeo's katana in Shangri-La as well as every gun you buy being automatically Pack-a-Punched.
At this point, yellow eggs will appear around the map. Help yourself find them by listening out as they apparently make a slight noise. Take them one by one in solo, or one each in co-op, inside the Apothicon. In the middle pool area are 4 pedestals at each corner. Place an egg in one and kill zombies to collect souls until a Gateworm hatches and appears.
There are many possible spawn points for these eggs. Click this annotation to view currently known spawns.
After collecting the Gateworms, start making your way around the map. The worms will emit sonar beeps to help you find where a Rune is located. When near, interact with the area or object. If in the correct spot, the Gateworm will be placed down, disentegrate, and a Rune will appear to be collected.
Return to the projector room in Kino after collecting all 4 Runes and each player needs to stand on a symbol. Doing so will then teleport you to the boss arena.
This step may be player-symbol specific. If so, if on solo, stand on any symbol as you would have collected them all. If in co-op, players need to stand on the symbol (or one of) that they collected.
Across from where you spawn in will be a book on a podium. Interact with it and it will open up and turn to 4 different pages. On each page is a symbol. Memorize or take note of the symbols and the order they appear in. Head back across to where you spawned and a symbol will be floating. It will change periodically, so when you see the first symbol that appeared in the book, interact with it to lock it in place. Do this for the second, third, and fourth symbols.
After getting all four correct, your first boss fight will kick into play. Margwas, Furies and Parasites will appear along with zombies in element-specific waves. Killing all current Margwas will activate the next wave. Once finished, the Summoning Key will lower to your level in the middle of the room. Grab it to be teleported back.
With the Summoning Key in hand, visit each location in the map and throw it at a certain object. Successfully hitting it will cause a large white glow and an item to appear (Monkey bomb, teddy bear, rocket, etc.) and the Key will be launched back at you. If you miss, the Key will respawn on the ground near you.
Throw the Key at the following objects:
After hitting the last object, the Key will disappear. Head back to the Kino teleporter to go back to the boss arena for the final fight.
Back in the arena, the Shadowman will appear across from your spawn, above the book. Having a shield, you'll need to take that down. At each corner of the room are pedestals with green flames. Interact with one to place the Summoning Key and kill enemies to collect souls. Enemies can be killed from anywhere in the room and souls will travel over, so you don't need to be near the pedestal.
Once enough souls are collected, grab the Key and throw it at Sophia who is floating near one of the pillars by where you spawn in. Sophia will fire a laser at the Shadowman, destroying his shield.
Like in Shadows of Evil, shoot the Shadowman to have him transport to another location. Keep shooting him at each location until he eventually is forced into the Apothicon's mouth. Quickly interact with the book on the podium, which will fire a laser at the Shadowman, destroying him. All enemies will despawn, the custcene will roll, and the Easter Egg is completed!
This fight can be done in the first wave. If not, elemental-specific waves will occur like before, just repeat the soul-collecting as needed.