Serial killer

Through years if experience torturing and killing, you have gained a thorough understanding of the anatomy of various humanoids, beasts and magical creatures. Through this knowledge you know where to strike to hit vital arteries, causing the creature to bleed out.
When you hit a creature with a slashing or piercing melee weapon, the creature must make a Constitution saving throw. If it fail, it pours blood from the wound and takes 1 strength score damage. The DC for the save is equal to 8 + proficiency mod + str or dex mod. Str or dex mod is determined by which stat was used to make the attack.