Before you can begin the Main Easter Egg, you must first unlock the Pack-a-Punch. You will also need to have a Death of Orion on at least one player.
There are four sub-quests you must complete in a particular order to finish the first section of the main quest. Each quest centers around one of the four Gods, and are often found within that God's Temple.
Collect the Ingredients
Inside the temple, there are small walls of skulls throughout the area. One of these skulls will have a blue symbol on its head. While looking at the skull, you must activate your special weapon, then interact with it. If done correctly, the skull will pop out of the wall and can be picked up.
Next you will need to take the skull and the Death of Orion to the Flooded Crypts. A grinder will appear in one of severl locations in the Flooded Crypts. Place the skull inside of it and shoot the grinder with the Death of Orion, fully charged, three times. Once completed, the skull will have been turned into bonemeal. Pick up the bonemeal to proceed.
Inside the main Arena will be two large pyres. You must lure a Gladiator near one of the pyres and have them throw their axes through it. If done correctly a chunk of wood should fall off. Pick it up.
Head to the basement of Odin Tower and look above the large cauldron in the center of the room. Interact with the cauldron to place the wood piece into one of the chains hanging from the ceiling. Wait two rounds. After two rounds, pick up the now burned wood.
In order to retrieve the poo, you must get minimum Crowd Affinity. Go to the arena with minimum Crowd Affinity and eventually the crowd will throw a pile of poop into the Arena. Simply run over to the pile to pick it up. You will take damage and lose points.
Mix the Ingredients
With all three ingredients in hand, proceed to the basement of Zeus tower. Next to the Blood Bath, there will be a large bowl on a counter. Interact with the bowl to place the three ingredients inside.
Wait 2 rounds.
Once the ingredients are fully mixed, the resulting mixture will have a green gas cloud above it. Pick up the mixture.
Open the Portal
Take the mixture to the basement of Danu Tower and place it between the trunks of the two trees.
Wait 2 rounds.
Once the mixture is ready, it will begin giving off the green gas cloud again. Lure a zombie over the mixture and kill it with a gun upgraded with Fire Bomb to cause a small explosion. Once done, there will be large blue cracks in the ground where the mixture once sat.
Have all players stand on the crack for several seconds. Eventually the players will be transported to a new instance of Danu Tower.
Kill the Red Sacs
All players should look up to the tree and begin shooting the cluster of red sacs. Once all four sacs are popped, a free Max Ammo will appear and the staircase to the next floor will be opened.
Repeat this process twice more on the ground and top floors to complete Danu's quest.
Once Danu has been completed, four bull heads will appear in one of several random locations across the map. You must shoot each of these heads with the Brazen Bull to summon a Gladiator. Kill each of these Gladiators and then proceed to the top floor of Ra Tower.
The black Obelisk in the center of the room will now have a blue symbol etched into it. All the players must interact with the symbol to be transported to a new instance of Ra Tower.
Next, you will need to watch the obelisk and remember the four symbols that appear and which order they appear in. Each symbol corresponds with a Special Enemy. You must kill each of these enemies in the order that their symbols appeared on the Obelisk. If you kill an enemy out of order, you will be removed from the instance, and must finish the round before being allowed to try again.
This challenge must be completed twice.
After Ra has been completed, a symbol will appear on the pedastal in the center of the Challenge platform. When all four players interact with the symbol, they will be teleported to a new instance of the Temple/Crypts.
Four poles will spawn within several windows near each of the four temples. Shooting these poles will cause them to rise up towards the ceiling. Once all four poles have been shot, proceed to the Arena.
The four poles can be found on the edges of the Arena near each of the temples. A blue ring can be found on the ground encircling a pole. You must kill enough enemies in the ring using the Kill-O-Watt Pack-a-Punch effect for the pole to become charged.
When a pole is fully charged, one of the Challenge Bowls in the center of the arena will be brightly lit.
Once all four poles have been lit, you will be given an unlimited specialist weapon and need to survive an onslaught of Gladiators and Brawlers.
Once the unlimited specialist weapon round has been survived, nine blue symbols will spawn in the lower section of the map. You must use the Death of Orion to shoot each of these symbols at a particular angle. If shot properly, the symbol will remain illuminated.
Once all nine symbols are lit, proceed to The Pit next to the Brazen Bull. Have all players stand on the grating nearby for ~20 seconds. You will be teleported to a new instance of The Pit and must survive multiple waves.
After surviving, you will be able to return to the normal map and a red portal will have appeared within the Arena.
After surviving several special infected attacks, a large elephant will spawn. Shoot the armor on its sides to destory it. Once the armor is removed, attack the elephant's head to finish it off.
Once the first elephant is defeated, a second will appear out of the other large gate. Kill this elephant in the same way to complete the bossfight and finish the Main Quest.