Thunderstorm

Your hands raise to the sky as gray clouds gather above you in a square area 50 feet long and wide. Any creatures under the storm must make a Dexterity saving throw each turn the spell is active. A creature takes 10d8 lightning damage on a failed save, or half as much damage on a successful one.

The storm can be moved 10 feet every round while the caster is concentrating.

Casting it using a higher level spell slot increases the damage dealt by 1d8 for each slot level above 6th.