For me it’s also Harry being a horcrux.
When I first saw the chambers of secrets and I was 6 back then, and Harry started to talk to snakes and my mind went started to think “what if Harry is related to Voldemort?”
As I got older and I got into the series a bit more, I began to understand a bit more. The order of the phoenix confirmed it for me that Harry is somehow related to Voldemort, but didn’t know what. When I got to the half blood prince and it was revealed that Voldemort had split his soul into 7 pieces aka horcruxes and that the diary was one of them, it hit me that Harry was one of them. And then of course we all know that it was revealed in the last book.
