Sword of Life Stealing is a rare weapon in D&D 5e. It requires attunement.
Weapon (any sword), rare (requires attunement)
When you attack a creature with this magic weapon and roll a 20 on the attack roll, that target takes an extra 3d6 necrotic damage, provided that the target isn't a construct or an undead. You gain temporary hit points equal to the extra damage dealt.