the old demon king died my guess is the old white crow
and as far as the over all it left a lot to be desired,
the author made it too shoujoish and didn't explore the monster element of the manga.
if the author had developed hiyo's power along with the story then the manga would have been more enjoyable (she went from barely being able to produce little wings to wielding huge amounts of power in the end).
I was a little disappointed in the ending as well... I felt like there was a lot left undone, and unsaid.
Like the father, always seemed like he had great power, yet we never got to see him use it... nor find out more about his past.. but my guess is that rajeshv is right about him being the old white crow.