Hi, unlesses are annoying. My favorite trick to deal with them is to replace the term unless with the term "if not," since that's what unless really means. With this, the statement in question becomes H couldn't have defeated V if he were not the chosen one. This is a much easier conditional to understand, it diagrams as If not Co then not Dv, or as If DV then Co
Note that the initial statement doesn't mean that if you're the chosen one then you're guaranteed to beat Voldemort. It's telling us that only the chosen one can do it, not that the chosen one is guaranteed to succeed.
