Hi there,
Here are a few clip files i use for terrain modifications.
The curve.tgc file apply a simple and single curve to a fixed altitude range. The curve uses a power function so it's parabolic, just tune the parameter node to fit your desire (a value of 1 leaves the terrain unchanged, 2 applies a square curve and 0.5 applies a square root curve, any other value is possible but i recommend values above 0)
The curve_adv.tgc does the same as above, but with a circular curve. The shape parameter determine the radius of the circle. Any value except 0 is possible. Values close to 0 leave the terrain unchanged.
This function is more complex than the previous one, so it will probably be much slower.
The expand.tgc file modify a fixed altitude range by lowering or rising the max altitude (its action is similar to the clamp node in world machine)
The cut.tgc file just remove the terrain above a certain altitude.
To create more complexe curves, insert those nodes a few times with successive altitude ranges and different parameters.
To use those files plug your terrain in the "get altitude" and the "displacement" nodes and use the output use the displacement shader as the output.
If you need more explanations or exemples, just ask.