Thunderstorm Decent

Thunderstorm Decent is a 6th level spell that can be prepared in advance and left hanging till the final word is spoken. Five fingers of lightning descends from the sky striking an area of 20 feet chosen by the caster within line of sight.  Each creature in the line must make a Dexterity saving throw. A creature takes 13d6 lightning damage on a failed save, or half as much damage on a successful one. The lightning ignites flammable objects in the area that aren’t being worn or carried. 

(Note: Spell can Not be used indoors or underground.)