The King: The Eternal Monarch

Lee Gon (Lee Min Ho) is the King of the Kingdom of Korea, a parallel world in which the royal monarchy still exists. When he was young, his uncle tried to seize the throne, killed his dad and almost killed him. He then fled to the Republic of Korea thanks to a magical item. Lee Gon possesses the other half of this item and years later manages to cross over, where he meets Jeong Tae Eul (Kim Go Eun), a police officer.