To start the easter egg, you must build the Atomwaffe.
This can be done by activating and filling 3 generators around the map with zombies. These work the same as the Mob of the Dead hounds or Der Eisendrache dragons except it can take multiple zombies body at once.
At the main place next to Mule Kick
In the Juggernog tunnels
You can now build the Atomwaffe at the bench in the room after the power switch.
You have now access to the first of the 3 stones needed to complete the Easter Egg.
First you need to go to the Speed Cola area. You will hear electricity coming from the phone poles, shoot it with Atomwaffe then continue with the next one.
Once you've shot 3 of them, you can pick up the gold ingot inside the box and put inside the furnace, in the Double Tap area.
Wait a round and you will be able to get the key of one of the drawer in the Stamin'Up building, take the temple stone inside it.
You can now put it on the ritual table, deep down the mines.
By now, the ritual circles should have gone red and you can find the second temple stone floating above one of them.
Fill it with souls, follow it to the next circle, repeat this process till the temple stone fall on the ground.
It will now say that the stone is on fire, to cool it down you will need to find a bucket and fill it with water, and then you will be able to grab the stone.
Fill it with water at the last level of the mines.
You can now put the second stone on the ritual table.
You now need to grab the shovel behind the 2 tunnels of spawn and start digging site until you get a dirty blue coin.
After grabbing the dirty blue coin you have to repair 3 electric boxes.
You can now put the blue coin inside the railing system and get the last temple stone.
You can now put the last temple on the ritual table.
When you interact once again with the ritual table, the three will start fusing into the summoning key.
A little step where you have to protect the summoning key from spawning keepers will now start.
Once done, grab the summoning key, put it on one of the glowing red pedestal around the map and protect it from keepers once again. You have to repeat this step 3 times as there is 3 glowing red pedestal.
In the Double Tap room
Behind the rail system round
Up the stairs of the MG-trap / Speed Cola area
Now you can use the summoning key to open the door in the Juggernog room, beneath the bar.
To enter the Boss area, you must interact with the teleporter and link it, once done a timer will start, and you must be inside the teleporter when the sequence ends.
You will now be teleported inside the Boss fight area.
You now have to interact with switch and wait till it's charged. You will have to face a giant Panzerhound, and you will need to survive while Zombies, R.A.P.S and eventually Keepers will spawn.
Once the switch is charged, interact with it once again to damage the giant Panzerhound, and interact with it to make it charge once again.
You will have to damage the Boss 3 times in order to beat it and complete the Atonement main quest.
Once beaten, congratulation, you have beaten the Atonement easter egg, and can enjoy the ending cutscene.