You where placed into the adoption system when you were a newborn child having been found with no documentation or any idea of who your parents are. All attempts to find your parents failed and you ended up in the system as it is called.
Maybe you were adopted and live a happy normal life.
Maybe you ended up going from foster home to foster home until you reached their 18th birthday, and you were no longer the problem of the state.
Little did you or anyone else know that the reason all attempts to find your parents failed was because your parents were not human.
While you look human and even test as human from all medical tests you were actually not human at all.
You are in truth a magical creature, that live in their own magical world hidden from the world of human.
The magic that makes you seam human doesn’t last forever and as you near your 22nd birthday, your human form will give way and your magical form will come out and the truth about who and what you are will be revealed. Be it a centaur, a mermaid/merman, a fairy or any other of countless types of magical creatures, your body will transform, you may gain wings, tails or even have your gender change but there is no stopping it.
You will become a magical creature.