Backstabber’s Blade

Weapon (any finesse weapon), rare

This wavy-bladed dagger strikes true when your enemies are distracted. You gain a +1 bonus on attack and damage rolls with the weapon, and whenever you have advantage on your attack roll with it, your attack scores a critical hit on a roll of 19 or 20. You also gain this increased chance of a critical hit on any opportunity attack and if another enemy of the target is within 5 feet of it, that enemy isn’t incapacitated, and you don’t have disadvantage on the attack roll.

While these weapons are most commonly found in the form of a magic dagger, this magical ability can be imbued into any finesse weapon. A non-bladed version of this item is usually called an opportunist weapon.