Using your ability to bend limbs, break bone,  and create painful locks, you masterfully tie up your enemy dealing damage as you refuse to let go of your quarry.

On  a Critical hit, while the enemy is grappled, you find an opportunity, a raised arm, bent leg, exposed back, etc...  You attempt to break a bone on your enemy.  

If you choose to try and break a bone sacrifice a die of damage to apply effects.  Only one can be chosen on a critical per turn.

If you choose to break the neck the enemy, the enemy takes damage and must roll a constitution save, with advantage, or die.  (as long as target is corporeal)

If you choose to break the arm the enemy, the enemy takes damage and must roll a constitution save with disadvantage or receive disadvantage on attacks and only have the ability to make 1 attack per turn until one day has passed.

If you choose to break the leg  of the enemy, the enemy takes damage and must roll a constitution save with disadvantage or have mobility reduced to 1/4.

You can also to choose to not break a bone, but instead apply a critical lock on a limb, add +1 to your hit and damage for every successful attack as you continue to wrench on your enemies limb, pulling and snapping ligaments. Cumulative

On a successful broken bone or held lock receive advantage on further attacks on the enemy as you now know intimate knowledge on his weaknesses.