When Harry Potter cast his first Cruciatus Curse, he was successful. In doing so, he awoke the darkness in his head. It whispers, and it's never wrong. The darkness is hungry, and won’t be denied.
‘To go in the dark with a light is to know the light. To know the dark, go dark, go without sight and find that the dark, too, blooms and sings.’
Thanks to Fawkes, the Horcrux in Harry is destroyed by the basilisk in the Chamber of Secrets. But something destroyed needs to be replaced, and a soul bond is just the thing.
Not that Harry or Ginny plan to tell anyone about it, of course.
