So how many mortals ended up defeating Diablo in D III?

I understand that my question seems very strange. But I am very confused by one point. In the prologue of the book Storm of Light says that the victory over Diablo in the third game won one mortal soul. And the site said that it defeated a group of nephalem (Diablo history). How does it all fit together? Or is it something like the case of Wow and the guy in the red t-shirt?