You must start by finding 3 teleport coils. The locations are below.

Coil 1: Located at Cologne, in the middle, inside of a crashed airplane (My friends believe you can run in there with meds+vaccine to easily get it and run out).

Coil 2: Located at the train crash north of Ostsee Compound. It is inside of a train car.

Coil 3: Located inside of the underwater tunnel north of berlin. This tunnel can be seen in the map. You do have to go underwater (Get your diving up)

Next you'll have to find the teleportation device and install the coils into it.
This is located in the sewers in berlin. If you use your map you'll want to search the eastern part of the sewers.

Install the three coils and you're ready to teleport into the boss room and defeat the boss.
The boss is a retextured mega zombie with all zombie abilities. Not that hard to beat

Alright so once you're done fighting it, it will drop standard mega loot. Next you're now able to acquire the sword which is hanging on a case. You'll find that (While facing towards the stairs in the main room) on the left room above a fireplace.

All you do now is leave through the teleporter.

Have fun!