Burrowing concussion

Hollow: The creature's body parts are so thin hitting it at all is a feat in and of itself.

Multi attack: The Creature can make four slash attacks, any of those can be replaced with one Shockwave.

Slash: Melee attack+15 to hit, reach 10 ft., 1 target. Hit: 25 (3d6 + 8) slashing damage. 

Shockwave (above ground): The creature's body begins to vibrate as it lets out a bursting screech that deafens every creature within 70ft for five turns or until they take a short break. Creatures in the radius take a DEX 17 save or fall prone.

Missed opportunity:  When a creature attacks the Burrowing Concussion and misses by one the Burrowing Concussion can make an opportunity action of either slash or Shockwave (above ground).

Shockwave (recharge 5): While underground the ground above the creature in a 50ft radius explodes with a force that pushes any ground above it outward 50ft from the center and two fewer feet out for every foot away from the center. Creatures in the radius must make a Dex save for every two feet that they are in the radius. Examples: DEX 25 if they are directly above the creature, DEX 20 for 10 feet from the center (40ft into the radius) DEX 10 for 20ft into the radius. If they fail they take 1 force/bludgeoning damage (depending on the terrain) for every foot they are into the radius, Max of 50 Min of 1, or half as much on a successful save, taking no damage if they are on the outer 5ft.