Manga: DEAR. DOOR Vol. 3
DISCOVERY. BETRAYAL. SACRIFICE.
It's a literal Hell on Earth as demons use doors--unwitting human portals--to wreak havoc on the mortal realm. Kyungjoon has now eased into the role of door to the Demon King Cain, though he's at first reluctant to admit that he's developed feelings for the king. Cain also has grown fond of the human.
But their relationship might prove short-lived, as an angel's weapon has wounded Cain and time is running out for him. The only way to save the demon is for the weapon's shard to be removed, and the only one who can remove it is a skilled doctor-demon who's currently on the run.
Unknown to Cain, someone from his own camp is preventing the doctor from being found. Besides, to cure Cain, a sacrifice must be made...and it's a sacrifice that might prove too great to give.