Thunderous Storm

You close your eyes and mutter the enchantment. The sky darkens as black clouds appear above an area of your choosing. You then open your eyes and with the final words of the spell unleash a torrent of lighting. Any creature caught in the square must make a CON saving throw. On a failed save the creatures hit take 20d6 lightning damage and 20d6 thunder damage or half as much on a successful saving throw.