Posting a text guide here since there isn't one already

To start the Easter Egg, you must first unlock Pack-a-Punch by completing the power core. There are four parts for the power core that must be found:

• The main part of the core can be found next to a tree across the river from the UFO
  
• The next part if a piece of curved metal that can be found on the path near Quick Revive
  
• The second piece of curved metal can be found by the dead body near Staminup
  
• The third piece of curved metal can be found on the hood of the truck by Double Tap

Once all the parts have been obtained, the finished power core can be placed in the UFO and will act as Pack-a-Punch

Now that Pack-a-Punch has been unlocked, the UFO must be powered with souls. This must be done with a double-Pap'd weapon and requires around 50 kills (you will know you are done when the screen flashes)

Once the previous step has finished, all players will be teleported in front of Mule Kick with the ability to pick up a flashlight off the ground and a sound clip/text will play

The UFO must once again be filled with 50 souls before the screen flashes again

Note: there is no turning back once this step has finished, so make sure you are prepared

Once the previous step has finished, all players will be teleported to a new area, where you must run to the end, being chased by zombies and hellhounds. Once someone has reached the end, the screen will fade to black as a sound clip/text plays, and the game will end