"I think blood bound me," because it was strong, stronger than just magic alone. There were reasons that their history had rituals involving such, right up to human sacrifices. "And blood calls to blood, her magic and yours. It reacted to what you did."

And that it allowed his actions to tie into the spells of another? Suggested either they expected him to regain power (unlikely) or he was stronger than the one that laid the original spell in the first place.