Darkside was added in Patch 1.1 (Horde Mode Update) on the 8th of November 2019. Darkside is the map you play on in Horde mode, a 50 wave zombie survival gamemode. The map features a big barn, a windmill and a crashed Helicopter.
Darkside features two easter eggs.
Inside the barn, you can see a radio on a picnic table near the entrance. If you walk up to the radio and press E, the radio will play a song. There is a total of 5 songs, 2 of which are versions of the main menu theme.
If you walk inside the helicopter in the middle of the map, you can press E near the cockpit to "drive" the plane. What actually happens is a sound effect of a sad trombone will play.