Conan Exiles

Settings/Server Settings/General/Make Admin
Go to Admin Panel

Spawn Humanoid tab on the top middle
Search and spawn King Beneath
Kill with sword or kill with all spawned NPC button

Video @ 6:20

If you don't want to follow the guide where you spawn the NPC in and use the kill command, you can find the King Beneath in a location named The Barrow King, a cave at A 11 on the map to the North West. You will need demon blood in order to open the cave door and head inside to the boss fight. There are several ways to obtain demon blood. I suggest harvesting the Albino Bat at the top of the Tower of Bats (F 5 on the map), or undead hyenas that spawn around The Sinkhole (H 6 on the map) with a skinning knife. The boss has ~4k HP & long reaching attacks, but doesn't pose too much of a threat if you have heavy armour and a decent weapon. (Much easier than a 3-skull world boss like Rotbranch.) Good luck!