2024 rules reference for Sword of Life Stealing.
Weapon (Glaive, Greatsword, Longsword, Rapier, Scimitar, or Shortsword)
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'ta Construct or an Undead, and you gain Temporary Hit Points equal to the amount of Necrotic damage taken.