PS: This guide is only destinated for those who already has experience with dedicated servers setup and forwarded the game ports to run the server. Default Port 7777 Default QueryPort 27016 both UDP and TCP.

The official tool from the game: The Haunted: Hells Reach Dedicated Server Tool no longer works, this is why you will never see a dedicated server into the server's list, the reason is unknown, probably some configuration. This happened after the DLC release, the update broke the way servers work and you can't either start the dedicated server tool.
You can be able to make a local server (Listen) but will have extreme lag and delay for all other players.

I have found a FIX to be able to play with friends with another dedicated server file inside the base game but the server won't be shown into Server's List. That means people will only be able to play if all have the IP from the Server Owner. This is the only way from now, at Least will be possible to play without boring lags.

Instead of using the official tool wich no longer works, you will get inside the game files, there is already .BAT files to start a dedicated server and it works fine. The standard path is:
(It can be different if you installed the game on another Disk)
C:\Program Files\Steam\steamapps\common\the-haunted-hells-reach\Binaries\Win32

Inside this folder, right click on the "Haunted DedicatedServer Inferno 7777.bat" file and click edit to change some parameters like difficulty and map. It shoud be like this from now:

start HauntedGame.exe server TH-Temple?game=HauntedGame.TH_Game_Coop?InitialDifficulty=2?AdminPassword=12345?Port=7777

This will start the game with Easy difficulty and TH-Temple as map. Save it and close. Now open the .bat file and the game. To join the server press ' to open console (the key above TAB and under ESC) and type: open yourlocalIP:7777.
Example: open 192.168.2.6:7777
Now you can play with friends, just send them your public IP to join.



A small list of some parameters to set into .bat file to change difficulty and map, just replace it on the according parameter . (Make sure to close the server before changing parameters).

Difficulty:
InitialDifficulty=1 (Beginner) No Map Achievements, only Easy or higher
InitialDifficulty=2 (Easy)
InitialDifficulty=3 (Medium)
InitialDifficulty=4 (Hard)
InitialDifficulty=5 (Nightmare)

Maps:
TH-Canyon
TH-Excavation
TH-Fog
TH-Island
TH-Mine
TH-Museum
TH-Nightfrontier
TH-Santatown
TH-Steeltown
TH-Stronghold
TH-Temple

This guide still lacks of information and will be fullfilled with new informations as time goes. Anyway, the game is very old and hard to find parameters/commands. If you have any tips, let me know in the comments so I can add it here. Don't forget to like the guide so others can see it as well.