Leviathan’s Tooth

Weapon (club or greatclub), very rare
  Carved from the tooth of a leviathan, a giant sea snake, this huge club sports a thriving ecosystem of halophilic moss, barnacles, and seaweed. You have a +2 bonus to attack and damage rolls made with this magic weapon.
  Leviathan’s Might. As an action you can slam the tooth into the ground, or thrust it into a body of water, creating a 30-foot-cone shockwave. All creatures in the area must make a DC 16 Strength saving throw. On a failure, a creature takes 4d10 thunder damage and is knocked prone. On a success, it takes half as much damage and is not knocked prone. Once you have used this property, you can’t do so again until you finish a long rest.