Hi,
You can use an "Apply mask" to display a mask over a terrain (the mask is automatically converted into a color map and the color map is applied onto the terrain).
Please note that if your mask is black, the terrain will be black as well. To fix this, you can add a "Scale & Offset" node to slightly increase the values of the mask. You can also use a "Min & max values" node to set a minimum value to the mask.