Urannon's Thunderbolt

As an action, you command the skies to concentrate their energy into powerful lightning. When you cast this spell, your first turn is spent summoning the lightning, concentrating on the spell. 

If you are still concentrating on the start of your next turn, a thunderbolt strikes in a 15-foot radius, centered on a point within range that you can see. All creatures in this radius must make a Dexterity saving throw. On a failed save, a creature takes 6d10 lightning damage, and half as much on a success.