Combustion

As a reaction to being hit your body produces a violent burst of flame, burning the creature that damaged you. The creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 1d6 fire damage and none on a successful one. 

This spell's damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).