That's pretty simple actually; with the voronoi you make well, the voronoi noise (color adjust needed to get hard or soften edges of the 'cracks', the top right is a constant scalar with the width/scale of the cracks), then you displace that down with a displacement shader, then you add a compute terrain to calculate new normals and XYZ variables, then you can add a displacement variable that can move the walls more vertical or even overhangs, either by a single displacement value or by a power fractal.
If you build what you see in that image, you'll find out. The lower 2 compute terrains are not really needed.