The spell seems to be about right power-wise. But I wonder why the progression is so weird; 1x 4d6 from 3rd, 2x 4d6 from 7th, 3x 4d6 from 11th. Wouldn't it be 'neater' with total damage dice equal to/no greater than caster level; 1x 3d6 from 3rd, 2x 3d6 from 6th, 3x 3d6 from 9th, 4x 3d6 from...