Bonecrusher

As you deliver your attack, your enemy's skeleton
begins to fracture in hundreds of places.
You make a melee attack that deals an extra 4d6
damage. The creature struck must succeed on a
Fortitude save (DC 13 + your Str modifier) or
its skeletal structure becomes massively weakened,
and all rolls made to confirm a critical hit against
the target gain a +10 bonus. A successful save does
not negate the extra damage. This effect lasts until
the target's hit points are fully restored.