Heartripper (5e Feat)
From D&D Wiki
Your strength is matched only with ferocity with which you fight.
- Your unarmed melee attacks deal 1d6 damage.
- Whenever your unarmed melee attack would leave the target's hit points below threshold, the target is instantly killed - you rip it's heart! (Such heart can be used as i. e. alchemical ingredient). The threshold equals to your level plus your strength score.