Black Masked Terrain

[NewbieMapper]

Anyone know how do they put a dark terrain\area like in the map "Blair Witch" Or "Is AnyBody Out There?!"?.

I meant,a visible area that goes Black Masked when a unit leaves the area.

Things I Tried:
-I tried putting the triggers "Enable Fog Of War" And "Enable Black Mask".-

It still doesnt work...any ideas?