Minecraft wiki entry for Ultra_hardcore_survival, associated text: To play on Ultra Hardcore, you must first create a Hardcore world. There are two ways to make a Hardcore world a UHC world. For the first method, once the world has finished generating, open the pause menu, click the open to LAN option, and turn on the allow Cheats option, before clicking the "Start LAN World" button. Once this is done, exit the menu and enter the following command into the chatbox: /gamerule naturalRegeneration false. Once you have completed these steps, save and exit to the title screen, before returning to the game. For the second method, click create new world in Java Edition 1.16 or above, and set gamemode to Hardcore. Next, click game rules, and select natural regeneration to make it false. Once you have done that, click create world. Now you have successfully created your own Ultra Hardcore world.