Dagger of the Deep

Dagger, uncommon
  A green jewel sits like an eye in the handle of this dagger, giving life to the carved, bone cuttlefish that forms its grip.
  When you hit a target with this magic weapon, you can spend a bonus action to speak its command word. With a convulsion, an inky-black ichor is injected into the target who must make a DC 13 Constitution saving throw. On a failure, the target takes an additional 2d6 necrotic damage and is blinded for 1 minute. On a success, it takes half as much necrotic damage and is not blinded. The creature can repeat the Constitution saving throw at the end of each of its turns, ending the effect on a success.
  Once you use this property, you can’t do so again until you bathe the dagger for 8 hours in water.
  Rare variant: Attacks made with this dagger have a +1 bonus to attack and damage rolls. The DC of the saving throw increases to 15, and it deals 4d6 necrotic damage on a failure.
  Very rare variant: Attacks made with this dagger have a +2 bonus to attack and damage rolls. The DC of the saving throw increases to 16, and it deals 6d6 necrotic damage on a failure.
  Legendary variant: Attacks made with this dagger have a +3 bonus to attack and damage rolls. The DC of the saving throw increases to 17, and it deals 9d6 necrotic damage on a failure.