Subterranean. The thunderfist is always partially underground and can only move, or be moved by burrowing. Restraining, grappling or knockback effects will not affect this creature and it's movement never provokes attacks of opportunity. The thunderfist can occupy another creature's space and vice versa.   Stone Grip. The thunderfist can attempt to grapple a target using both it's action and any remaining movement on it's turn. The target must make a Dexterity saving throw DC 12, on a failed save the target takes 6 (2d6) lightning damage. The target must repeat the saving throw at the start of each of its turns, on a failed save the target takes another 6 (2d6) lightning damage. On a success the grapple ends. The thunderfist cannot move or take actions while grappling a target.