Sword of Life Stealing

Source: D&D 5.2 SRD Magic Items

Weapon (Glaive, Greatsword, Longsword, Rapier, Scimitar, or Shortsword), Rare (Requires Attunement)

When you attack a creature with this magic weapon and roll a 20 on the d20 for the attack roll, that target takes an extra 15 Necrotic damage if it isn't a Construct or an Undead, and you gain Temporary Hit Points equal to the amount of Necrotic damage taken.