Heart Rob

Weapon (dagger), rare
  When this blade comes into contact with cardiac tissue, it immediately sends it into spasms, causing a heart attack and preventing pulmonary circulation. Such weapons are especially useful for preventing the spread of lycanthropy and vampirism by inhibiting the cursed toxins from spreading throughout a potential host’s body.
  You gain a +1 bonus to attack and damage rolls made with this magic weapon. In addition, the weapon ignores resistance to piercing damage.
  When you attack a creature that has at least one heart with this weapon and roll a 20 on the attack roll, you stab the creature directly in the heart. The creature immediately gains three levels of exhaustion and begins suffocating. If its heart is restored, such as with greater restoration or similar magic, it immediately stops suffocating. A creature is immune to this effect if it is immune to piercing damage, doesn't have or need a heart, has legendary actions, or the GM decides that the creature is too big for its heart to be punctured with this weapon. Such a creature instead takes an extra 4d6 piercing damage from the hit.
  Very rare variant: Increase the bonus to + 2, and the extra damage to 6d6.
  Legendary variant: Increase the bonus to + 3, and the extra damage to 8d6.