This is sort of an idea and some what half baked but the basic concept is as followed.
Your mother had been very sick before you were born, and in the darkest hour a demon appeared to your father eh/she/it spoke the truth and offered to heal your mother for the price of the first born body and should once she reached maturity
Now the father thinks he is getting something for nothing as he is 100% medically unable to have children, agreeing his wife miraculously get better followed by pregnancy and your birth your birth. Your whole childhood is odd, your never sick you heal any wounds very quickly and then there are your parents who spoil you outlandishly especially your mother always has this odd sadness about her when she looks at you.
When your birthday comes, it comes for you. And your life takes a turn for the bizarre
This would be NC I would even prefer extreme, more fore the sake of the possibility. Look at my o/o for guidelines and I would of course be happy to discuse any added ideas.
What is very important to me is that I find somebody descriptive
PM me :)