You have trained to the extreme with a specific weapon, and you know how to use it to be precise - and to strike devastating blows.

Choose a weapon. You gain proficiency with that type of weapon if you are not already. Whenever you make an attack with the chosen weapon, you may choose to add half of your proficiency bonus, rounded down, to either the weapon's attack roll or to the weapon's damage. You must choose before you know if your attack was successful or not.