The Newest Assassin
Assassin's Creed II x Harry Potter
Harry gets captured by Voldemort in the cemetery. Rather than kill him, Voldemort uses a special magical mask on Harry, causing Harry to relive the life of an ancestor whom Voldemort had chosen. Voldemort was seeking a long lost and incredibly powerful artifact called an 'Apple of Eden', which Harry's ancestor had come into possession of it back in the early 1300s. Voldemort watched the memories by copying them from Harry's mind (unlike Harry, Voldemort doesn't get the Bleeding Effect).
Using the Bleeding Effect to help him escape (late August) before the final memory reveals where it was hidden, Harry steals the mask and goes in search of the Apple himself. He finds it where it had been hidden (Stonehenge). He returns to the Wizarding World, who'd been going frantic with his and Cedric's disappearances. Though St. Mungo's did their best, they were unable to heal his 'mental trauma' and were stumped to what it was or what caused it. Despite their best efforts, word of Harry's injuries and mental instability was leaked to the public, along with his claim of Voldemort's return which Fudge dismissed as 'delusions of insanity'.
While at Hogwarts for his Fifth Year, Harry tests Ron and Hermione to see where their opinions lay: Assassin or Templar. Much to his distress, they were more opinionated towards the Templars. They thought that while free will was important, controlling the masses to prevent the rise of potential future Dark Lords was more important to the safety of the people. Harry tries talking to others in Hogwarts, regardless of House affiliation or age. He realizes that nearly all of them agree with the Templar mindset, in one form or another.
Angry at the WW and seeking to discover if the Templars had penetrated magical society, Harry begins rigorously training his body. He regularly takes trips into his ancestor's past to rediscover his memories, eventually unintentionally getting a Sync Nexus. He builds his own Hidden Blades using a combination of magic and ancient designs.
Once the OWLs are finished, Harry leaves Hogwarts to commit himself fully to his Assassin training and tries to break all ties with the Magical World until he's ready to face it again.