You have trained for many years, and can make precision cuts from any distance. Weather your 50 ft, or 2ft, if you can surpass your foes armour class you an wield the dagger like a needle (e.g. piercing a tendon, slicing off a finger). 

• You gain advantage on any attack made with a dagger
• Rolling a 19 or 20 results in a crit
• Pin-point accuracy
• Automatic proficioncy with weapon