A bright streak flashes from your pointing finger to a point you choose within range and then is striked by a powerful lightning bolt. Each creature in a 20-foot-radius sphere centered on that point must make a Constitution saving throw. A target takes 8d6 electricity damage on a failed save, or half as much damage on a successful one.

The lightning bolt is a cylinder of 5ft radius but the electricity spreads on the ground to 20ft of radius

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.