Requester: myoneday
The White Demon
https://www.asianfanfics.com/story/view/1091355/

In a time when people still believed in dark curses and demons walking among them, a prince was born. He was Jiyong's first heir and after Silla's law, he will become the next ruler. But things don't always end as they were planned. The baby was cursed. To save his life, the king decided to announce his child was a girl and raise prince Luhan as princess Luna. During one of their trips in the country side Luhan is kidnapped and saved by a 11 years old boy, named Sehun. Impressed by his abilities, the king hires him as Luhan's private guard. Less did he know that Sehun wasn't human, nor his real intentions.