Dunno why I missed posting this, but better later then never
This map took me around 500 hours to create and turned out pretty good!
Features a full on Easteregg with a fully scripted and animated boss, toggleable flashlights, playable organ and so on
The huge building is based on an old phosphate factory near me, where movies like The Hunger Games, Monuments Men or Inglourious Basterds were filmed.
Enough talk ... here are some pictures, a youtube video with the boss battle starting @ ~ 35min. and a link to the map on the steam workshop:
https://www.youtube.com/watch?v=oku5KjlEsQk
https://steamcommunity.com/sharedfiles/filedetails/?id=1197200650
Some reference pictures: