Ball Lightning

You manifest a ball of lightning in the shape of a 5ft diameter sphere and send it careening forth in a direction of your choice. The ball travels in a straight line moving through all creatures, gaining speed as it does so, until it has traveled up to 100ft from you, after which it stops moving. At the end of your next turn, or if it hits a wall during its movement, the ball explodes. Each creature within 20ft of the sphere must make a Dexterity saving throw taking 3d6 lightning damage, or half as much on a successful saving throw

Additionally, each creature the ball passes through, or that comes within 10ft of it during its move or while it is stationary, must make a Dexterity saving throw, taking 2d6 lightning damage, or half as much on a successful saving throw. 

The ball of lightning ignites flammable objects that aren't being worn or carried.

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