Lance of the Crab Knight

Weapon (lance), rare (requires attunement)
  The chitin of the giant centenary crab is some of the hardest found in the ocean, formed under the extreme pressure experienced thousands of meters below sea level.
  Crab Knight. You can cast the phantom steed spell. The steed always takes the form of a giant centenary crab, which has a swim speed equal to its walking speed. Once you have used this property, you can’t do so again until the following dawn.
  Clawful. As a bonus action, you can speak the lance’s command word. The missing half of the crab’s claw appears as a spectral, shimmering force for the next minute. As part of the summoning action, and as a bonus action on subsequent turns, you can cause the spectral claw to clamp shut when you hit a target, dealing an additional 1d10 force damage and grappling it (escape DC 15). While you use the lance to grapple a creature, you can’t use it to attack another target. You can release a grappled creature with a word, no action required.
  Hydrodynamic. Attacking while underwater doesn’t impose disadvantage on attack rolls you make with this weapon.
  Very rare version. You have a +2 bonus to attack and damage rolls made with this magic weapon.