You can do this in a couple of ways.
You can use a distance shader which blends in a terrain over distance. The distance is measured from a camera which is connected to the distance shader. The direction the camera faces is the direction the distance is calculated, in case you use Z-depth.
You can even create 2 z-depth distance masks, one as the reverse of the other, multiply those with a "multiply colour" function node to get a narrow straight band as a mask.
You can also create a camera high in the sky, have it face down 90 degrees, let te preview render finish, create a painted shader and paint the mask.
Third, you can create a mask in photoshop, load it into an "image map shader", set projection to "Plan Y" and change the size from 1m to something bigger.
The coordinate input allows you to move the mask over the terrain.
Basically all of this can ben found easily on these forums as there's a wealth of information and a search function too.
Guess I need to hurry up with Oshyan to get that FAQ posted here as these questions keep on showing up.
It would be better if all this information is more readily and compactly available.
Cheers,
Martin
