Hello sniperz,
maybe you want/need some more information.
If not, I am sorry to bump.
I assume you are using DarkPlaces.
First of all, to get the needed .ent file of your map, please start the map and type this in the console:
sv_saveentfile
(be careful, existing ones will be overwritten)
The file will be in your maps folder.
Now you can edit (with notepad for example), your .ent file
and add the fog line from Nahuel in it (in the end of first block).
If you want to know more about the values, here is the description:
fog : set global fog parameters (density red green blue [alpha [mindist [maxdist [top [fadedepth]]]]])
Have fun with your spookie fog.

Seven