Asa held a fraction of each of our sins. It was kind of complicated for me to wrap my head around, but basically, Asa was the ninth Nephilim ever born. The eight of us cursed sons came before him. As he drew his first breath, a piece of our curse flowed into him as well. Only a fraction, but it was enough for our blood to recognize Asa. Enough for the curses in us to recognize the one in him.
