In the material you basically need this:
the output of the SphereMask is your mask. Probably neet to 1-x and then multply it with your ground shader parts that you want to darken.
Hardness is how hard your shadow is.
Radius is the radius of the shadow.
PrimeLocation is the parameter you set with the blueprint. (X Y Z and the alpha is kinda wasted, but you could use it for something else. You can set a vector parameter instead of a float parameter, with a different blueprint node.)