Thousand Cuts Katana

Greatsword, very rare, requires attunement by a character with proficiency in scimitars
  While attuned with this weapon, you are considered proficient with it, even if you are not normally proficient with greatswords. This weapon has the finesse property. While wielding this weapon and have the sneak attack feature, you gain an additional use of the feature per round. You have twice tha amount of attacks with this weapon per turn. For example, if you have the Extra Attack Feature which grants you one additional attack when you take the attack action, you can attack 4x per turn, instead of 2x per turn, Attacks with this weapon deal an additional 1d6 poison damage, but any attack with this weapon deals half damage to the target due to it's razor edge and light weight lessening the momentum of the blow.