Bonecrusher

3rd-level / Stone Dragon

You deliver your attack, and your enemy’s eyes jerk wide open in panic as his skeleton begins to fracture in hundreds of places.   As part of this maneuver, you make a melee attack. If your attack hits, it deals an extra 4d6 points of damage. The creature struck must succeed on a Constiution saving throw or its skeletal structure becomes massively weakened. While weakened in this way any attacks against the creature critically strike on a 17, 18, 19 or 20. When the creature has been critically struck once the weakening ends, it also ends after 1 minute.