Valheim: set dedicated-server difficulty and world modifiers

Valheim dedicated servers accept difficulty presets and individual world modifiers in their startup file. Iron Gate documents -preset, -modifier and -setkey as server startup arguments.

Prepare your startup file

Edit a copy of start_headless_server.bat on Windows or start_server.sh on Linux. Steam updates reset the original file, so launch your customised copy directly. Keep a copy of your world and previous configuration before changing them.

Set the preset before its exceptions

Choose the base preset: Normal, Casual, Easy, Hard, Hardcore, Immersive or Hammer.
Add individual modifiers afterwards. A preset overwrites previous modifiers, so its exceptions must follow it.
Add a -setkey switch if needed. The manual lists nomap for the no-map modifier.
Save the file and start the server with that configuration. The manual says to stop a running server with Ctrl+C in its command window.

A combination following that order is -preset hard -modifier raids none: the Hard preset followed by the no-raids exception. Choose the changes your group actually wants.

Documented modifier values

Combat: veryeasy, easy, hard, veryhard.
DeathPenalty: casual, veryeasy, easy, hard, hardcore.
Resources: muchless, less, more, muchmore, most.
Raids: none, muchless, less, more, muchmore.
Portals: casual, hard, veryhard.

The additional -setkey values are nobuildcost, playerevents, passivemobs and nomap. This is a list of documented options, rather than a recommendation to enable them all.