Caustic Wave

You hold up a vial of acid and chant arcane words. From your palm rushes forth a wave of acid. 

All creatures within this 15 ft cone must make a Dexterity saving throw against your spell save DC. They take 6d6 acid damage immediately and an additional 2d6 on the start of their next turn on a failed save or half damage and no additional damage on the start of their turn on a successful one. 

At Higher Levels: when you cast this spell using a slot of 4th level or higher both sets of damage increase by 1d6.