Impetus Infitialis Magicae

Harrier Potter, defeated Voldemort not by dint of magic skill, but inherent anti-magical aura causing Voldemort's ritual to blow up in his face. Now separated at birth and trained for the next decade to the be the next mage killer of the Unspeakables, Britannia's first and last line of defense against the Outsiders and magical anomalies, Harry must try to survive what is to come.