So, during the events of Changes, Harry discovers that the ritual used in this book was (most likely) a Bloodline Curse, with the first/main clue being how it kills the victims- ripping their heart out. One problem with that- given that it requires sacrificing a relative in order to eliminate everyone above them in their family tree, how could Harry have been targeted?
It may have been a variant of the Changes ritual, where one doesn't need to sacrifice a relative, but pays for the convenience with not as much power—hence why it stops at the target.
Indeed, Vadderung explicitly says that the ritual is essentially a low-powered version of the bloodline curse.
