It all starts with a goodbye

'Opposites'

Harrison was opposite to his sister- Hope- in every way: Where she could barely control her magic, he could summon things when he was only one; where she was trained by tutors to defeat the dark lord, he was in the past getting trained by the founders; And where she was giving her life for the light, he became a new sign of hope for the dark.

or

Wrong G/BWL Au, Harrison time travels to the past to grow up with the founders only to go back to the future a day before his first day at hogwarts. He decided that the dark side shares more of his ideas than the light side and eventually even finds love in the man his sister is trying to destroy.

(This sounds so sappy, I promise it's not.)