When you take the attack action on your turn, you can replace one attack with a use of your breath weapon. Either a 30Ft line, 5Ft wide or a 15Ft cone. Each creature in effect much make a Dexterity saving throw (DC12). On a failed save, the creature takes 1d10 fire damage, half on a successful save.
The damage increases by 1d10 at 5th level, 10th level and 15th level.
You can use your breath weapon a number of times equal to your proficiency bonus, and you regain all uses when you finish a long rest.