The final battle against the Shadowman begins! Time to show him what you got.
Step 1: Place the Summoning Key in one of the green glowing standards on the upper level walkways.
Step 2: Charge the Summoning Key by killing zombies around it (watch out! Panzer Soldaten and Margwas will also spawn!). Do this untill the Key begins to shine a yellow colour. Interact with it and you will be able to pick it up, taking the place of your weapon. Now throw the key on Sophia's spirit, floating in the middle of the room. Upon doing so, it will release a red laser beam which destroys the Shadowman's shield.
Step 3: You can now damage the Shadowman by shooting on him. While doing so he will randomly teleport around the altar. Keep shooting on him untill he is in the mounth of the great Apothicon Overlord statue at the back of the arena.
Step 4: Now quickly, interact with the Kronorium book, laying on the altar. If you are too late, you have to redo the previous step untill the Shadowman is in the mounth again.
If you succeeded, you have finished the Easter Egg! Congratulations, the Cycle will continue!