It is time for the final bossfight! Prepare well, do any last-minute upgrades, perks and grab anything else you need as there is no going back. When you are ready, enter the Dark Aether and head to the Nacht der Untoten bunker. You'll find an anomality of Orlov's spirit, interact with it for a small cutscene.
After this, you will exit the Dark Aether and re-enter the normal world again. Interact with Orlov's family picture dropped on the floor of Nacht der Untoten to start the bossfight.
You will teleport to the Particle Accelerator room where Orlov waits for you. He recognises his family photo you give him and says he wants to shut down the Particle Accelerator.
Your mission is to survive and protect Orlov into doing what he must. A lot of zombies and Megatons will spawn in. Killing the Megatons will give you max ammo drops. Depending on with how many you are, one is advised to protect Orlov, while the others should kill the zombies and Megatons. Orlov will move around the Cyclotron, so be sure to follow and protect him. Once he has succeeded, the screen will turn purple and white.
Note: It is recommended to also read the next step before initiating the Boss Fight