Sharktooth

Weapon (longsword or greatsword), rare (requires attunement)
  Fashioned especially for the Hammerhead Knight, this blade caused its victims to bleed, allowing the knight, with its superior sense of smell, to track a would-be assassin back to its lair. You have a +2 bonus to attack and damage rolls made with this magic weapon.
  Maim. When you hit with a melee attack using this weapon, you can speak its command word as a bonus action, causing the teeth-like protrusions to rip through the target. Roll a d4 and consult the following table:
  d4 - Effect
  1 - No Additional Effect.
  2 - Hamstring. The target’s speed is reduced to 0 until the end of its next turn.
  3 - Rend. The target has disadvantage on attack rolls until the end of its next turn.
  4 - Sever. The target begins bleeding for 1 minute. Each time it takes bludgeoning, piercing, or slashing damage, it takes an additional 1d4 damage of the triggering type. The target can use an action to make a DC 15 Wisdom (Medicine) check, stopping the bleeding on a success. The Maim property can’t be used again until the following dawn.
  Hydrodynamic. Attacking while underwater doesn’t impose disadvantage on attack rolls you make with this weapon.
  Set of the Hammerhead Knight (Very rare). While attuned to both items of the hammerhead knight (the helmet and sharktooth), you gain advantage on Dexterity saving throws against effects you can see or smell and you can’t be surprised. In addition you can, as an action, grow gills and morph your legs into a tail (or vice-versa). You gain a swim speed of 40 feet (if not already higher), your walking speed drops to 5 feet, and you can breathe both air and water.