Mr. Weird Yin Yang

My name is Yu Kaixin. Originally, I could not be born due to dystocia. My grandfather used some special methods to bring me into this world. On the second day after I was born, my grandfather mysteriously disappeared. Before leaving, my grandfather and my father once said that I must go back to my hometown when I was 18 years old, because there was a great disaster when I was 18 years old, and it was also a taboo. It was also from the time when I was about to turn 18 that I slowly got involved in one weird thing after another. I saw things that many people couldn't see. And about the catastrophe when I was 18 years old? What exactly is it?