True Dragon's Breath

You touch a willing creature and compel it to breathe a rolling wave of ancient elemental magic from its mouth, provided it has one. Choose acid, cold, fire, lightning, or poison. Until the spell ends, the creature can use an action to exhale energy of the chosen type in a 30 foot cone or 30 foot long, 5 foot wide line (it chooses each time it takes the action). Each creature in that area must make a Dexterity saving throw, taking 12d6 damage of the chosen type on a failed save, or half as much damage on a successful one.