how do you make a black abyss around an island or town like in fuma.

Your best bet is here, probably:

www.doanthenado.com

Easy way is just to apply a black texture to everything around the town in the map editor. Another way is to overwrite the slope textures to just be black, but that might result in things you don't want (like you wouldn't be able to have mountains and such in the town). For making the fog you can either use the cloud VFX or just apply fog to the town using the script commands for weather in the towns Enter Map script.