Ao no Exorcist

Rin and his classmates Izumo and Shima are sent for their first mission as Exwires to a coast town aflicted by the presence of a Kraken. Izumo stumbles upon a boy whose father set to face the Kraken six months before and is still missing. The boy plans to enact his revenge against the creature as he believes it killed his father, but he and the Exwires find out what really happened when they encounter it, much to their surprise.