Every time you cast a spell above 1st level you have a chance to "surge".  Casting leveled spells causes a stir in your...bowels. Roll a d20, 1-5 you surge. A cloud of noxious gas errupts from behind in a 10ft cone behind you. In creature in that radius must make a constitution saving throw DC15 taking 2d10 damage on a failed save, or half as much a success.