Looking for Minecraft Mod?

The mod is supposed to add a kind of zone / border that is round if possible and creates a kind of fog when you are inside it. It is also said to cause damage. (As in Fortnite or Pubg)

As far as I know, it only works with a square boarder, i.e. Vanilla.

No mod would be known to me

This can be set up with commands. Google Worldborder command. Here is a link with a list of options: https://minecraft-de.gamepedia.com/...orldborder

Check out Minecraft Forge.

Check out which youtube videos about mc mods, you will definitely find it. I used to do it that way.