Putting Fog Around My Map

I have a mountain range for a map. Since it can’t go on forever, I want to put fog on the edges of my map so players can tell where the map ends. What is the most efficient way to go about this? Halos? Or is there another way?

i am guessing your map means terrian… you could place a texture of fog on the border… but placing fog to show where your ‘map’ ends isnt the right way… you need to rise your terrian border higher so the player dose not walk overboard… just thinks its the high mountain he cant climb…