How to fade out the borders of a map?

Hi guys, I was wondering how I would approach fading out the border/edge of a map naturally.
For example, I’ve seen some games use a bright white light or a fog effect to represent an invisible wall. How could you do this in Unreal? I know how to implement an invisible wall but have no idea how to add this effect!
I know you could use a building or other environment objects to block the player’s path however I was just wondering how you would go about doing this if that wasn’t an option.
Thanks!