Harry Potter makes a deal with a powerful demon to escape the Dursleys when he's young and gets transported into the world of Kuroshitsuji (Black Butler) where he becomes a reaper, one that is later known as Undertaker. Undertaker is later captured and would be executed by the other reapers because of his crimes (experiments).

Just before he's killed, he summons the demon. Another deal is made, Harry de-ages and escapes back to his world, to the moment right after he made the original deal. He kills the Dursleys, made it seem like it was the work of a random killer who escaped and pretends to be traumatized when the police finally comes. He gets taken to an orphanage and grows up there until the Hogwarts letter comes.

He goes to Hogwarts and basically wrecks the place, traumatizing teachers and students alike.

Things to keep in mind:

- The demon must be extremely powerful but must also have a soft spot for children. For example, Alcor the Dreambender.

- You can either let Harry have long white Undertaker hair when he goes to Hogwarts or normal Potter hair, it doesn't really matter.

- Undertaker-Harry is creepy and creeps everyone out with his eccentricities and quirks. His trunk should be a coffin and he should be able to summon his scythe.