Weapon (Glaive, Greatsword, Longsword, Rapier, Scimitar, or Shortsword), Rare (Requires Attunement)
When you hit a creature with an attack using this magic weapon, the target takes an extra 2d6 Necrotic damage and must succeed on a DC 15 Constitution saving throw or be unable to regain Hit Points for 1 hour. The target repeats the save at the end of each of its turns, ending the effect on itself on a success.
Source: SRD 5.2.1 · CC-BY-4.0