To form the crater shape use a spackle (US) or polyfilla (UK), the latter being actually better as it is cellulose-based and does not shrink or warp while drying. Note that my projects that involve a lot of water-based spackling paste are usually constructed on a plywood base to prevent warping
Cover the whole thing with a PVA glue and sprinkle with fine sand:
When it is all dry, shake off excessive sand
Spray it all with a black paint
Drybrush it all with a grey paint
Glue some static grass to the base's rim
Blend the line between grass and sand with some black paint
To create a metropolis that would make Marinetti and his 1920s Futurists proud, all you need is a lot of polystyrene foam and a window-shaped hot metal branding mark. Here comes "The City, The Crowd, The Machine" of old avant-garde dreams in 28mm scale. »
Folks at Games Workshop love to put skulls all over anything. We in Central Europe understand their liking. It comes from an ancient Christian invocation: Terminus nulli cedo - The end (Death) bows to no one »