Essence Theft (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast at least one spell
You can use your necromantic powers to steal the essence of creatures. You gain the following benefits:
- When you reduce a hostile creature to 0 hit points with necrotic damage, you gain temporary hit points equal to your spellcasting ability modifier + your character level (minimum of 1).
- You can use your bonus action on your turn to change the damage type of any spell you cast to necrotic damage.