Deep Freeze

The fog you summoned instantly turns to ice. Each creature in a 20-foot-radius Sphere centered on that point makes a Constitution saving throw, taking 8d6 Cold damage on a failed save or half as much damage on a successful one.

The ice covers the ground in a the area of the Fog Cloud and turns it into Difficult Terrain for one minute.

When the ice appears, each creature standing in its area must succeed on a Dexterity saving throw or have the Prone condition. A creature that enters the area or ends its turn there must also succeed on that save or fall Prone.

Using a Higher-Level Spell Slot. The damage increases by 1d6 for each spell slot level above 3.