Hmm, careful with that capstone, it seems very easily abused. For example, if the ward had Diehard, he would essentially be invincible as long as he was within 30 ft of the Defender.

Instead, I would change it so that all of the damage done to the ward in excess of -9 isn't ignored, but instead is taken by the Defender. You might also make it so that the Defender is moved adjacent to the ward when the ability triggers, though that might not be necessary.