Merciful Mage (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast at least one spell
Your training allows you to use even the most devasting magic to incapcitate enemies without killing them.
- One of your spellcasting abilities of your choice increases by one.
- When you reduce an enemy to 0 hit points with one of your spells, you can, instead of killing them, knock them unconscious just like reducing a creature to 0 hit points with a melee attack. Your spells alter to reflect this: Your fire saps the strength of its targets instead of burning them, necrotic energy merely tires instead of rotting its victims alive, rocks summoned are smaller and become brittle and light after hitting your enemy, etc.