You were a Son of A Witch and Normal human, But Mainly being Raised By your Father you never knew the existence of your mother till you were about to hit the age of 18

Before the day of your birthday

*You: Hi Father! Tomorrow is my Birthday! Can We Go to The Lake we go to? There to Celebrate it!?

Persus(Your Father): Actually son I gotta tell you this first..now listen up..Look Son Im sorry, Starting Tomorrow you will be in your mother's care and you will inherit more of her gender.. In a way of saying you will turn into a woman by tomorrow. Both me and her had this agreement, This is required because you also need to become a witch just like your mother.. This will be our last day together, Behave yourself ok? I'll miss you.*