Arkhan's Tectonic Crush

Slamming your fists into the ground with the force of an earthquake, a blast of earth and sharp stones erupts from the ground. Each creature in a 40-foot cube on and below the ground originating from you must make a Dexterity saving throw. A creature takes 8d8 magical bludgeoning damage and falls prone on a failed save, or half as much damage on a successful one and doesn't fall prone. Creatures not on or under the ground when the spell is cast are immune to this damage.

A creature who is underground in the area of the square makes their save with disadvantage, and if they fail, are ejected to the surface and fall prone.

All terrain within the cone becomes difficult terrain, each 5ft square taking 1 minute to clear by hand.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d8 for each slot level above 5th.