I can help wit dat
for some really spooky effects try this
Make all your lights emmeanate from the ground up...making it appear dark outside, while allowing the player to see clearly
use Volumetric lighting, this is what produces a foggish appearance, you'd have to get Chris Toth to explain Volumetric lights, i still don't understand them
add decorations that are appropriate, don't add a colorful sign in the midst of a graveyard! use things like the monk statue, which looks really spooky,
follow these and a few other simple ideas and your map should be pretty spooky when you get done
as for making moutains, select cubes and build cubes or use the floor lofter when you use cubes, make them look realistic with sheets, cones, cylinders, and spheres, it won't look perfect but he more you experiment the better you'll get, have fun with these tips!!
Glad I could help!