Deathblow (Overkill Supplement)
From D&D Wiki
When you take your action on your turn, you can use your action to attempt a deathblow, a devastating melee attack which can kill a creature outright. Make a Strength check, or a Dexterity check if you can use Dexterity in place of Strength when you make an attack roll with the weapon you're using. You may add your proficiency bonus to the Strength check if you are proficient with the weapon you're using. The DC of this Strength check is 10 + 1 for every 5 full hit points your target has remaining. On a success, the target of your deathblow dies outright, and you may narrate exactly how you graphically slay your victim. You have advantage on this Strength check if your target is vulnerable to the type of damage your attack uses, if you are raging, or if your target is paralyzed, petrified, restrained, stunned, or unconscious. You have disadvantage on your Strength check if you have disadvantage on melee attack rolls for any reason, or if your target has resistance to the type of damage your attack uses. A deathblow attempt automatically fails if your target has immunity to the type of damage your attack uses, or if your target is immune to critical hits.
On a failed deathblow attempt, make a melee attack roll. On a hit, you deal normal damage. Damage from a failed deathblow may still kill your target, just not as spectacularly.