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.
Source: SRD 5.2.1 · CC-BY-4.0