Special thanks to PoolboyQ, McClone, Aqua and the SDK dev crew!
Music: Anubis - Instrumental metal music by Doverzen
!!!DISABLE OTHER TEXTURE MODS BEFORE PLAYING!!!
Installation: (shamefully copied from the official guide)
Right click on the game in your Steam library and select "properties," then navigate to the "BETAS" tab. You should see a public SDK branch.
Note that the SDK version is currently a separate program from the main game. To run it, start DUSK from your Steam library (using the green play button). A launcher should appear with "Launch DUSK SDK" as an option. Choosing "Play DUSK (singleplayer)" instead, or launching Dusk from a desktop shortcut, WILL NOT LAUNCH THE SDK. If there isn't a "modding" button on the main menu, you've launched the wrong version.
For GOG Galaxy:
Navigate to your GOG library and "manage installation" for DUSK. There you should find the option to enable beta channels. Enter the following password to access the DUSK SDK
After that you should have the SDK channel available, upon joining it the SDK will download automatically. To access the SDK you can right-click the game in library and select SDK. Those on Galaxy 2.0 will have to navigate to DUSK\SDK and launch via the respective batch files. dusk_win for Windows, dusk_tux for Linux
After you enabled the beta:
Locate "local" folder (Dusk\SDK\mnt\local) and put "Trespasser" file here from the archive.
Now it should look like this: Dusk\SDK\mnt\local\Trespasser
Then run the game (make sure that you're running dusk_sdk version! You should see "MODDING" button in the main menu), press "Modding" button, find Trespasser mod, check little box on the right side of the list, and press APPLY. Then press "CAMPAIGN" and "CUSTOM LEVEL". You should see "Trespasser" level in the list. Press it and, TA-DA! Enjoy the map.