Ao3 recommendation
Kingdoms Apart, yet Still Hand in Hand
Prince Hyunjin is to be married to Prince Christopher to solidify a treaty between their two kingdoms. While neither boy is happy about the arrangement, they plan on trying their best to at least be civil with each other. After all, their happiness comes second to the safety of their homes.
Despite this, they find themselves butting heads more often than not. When people around the castle begin dying, and their group dynamics begin changing rapidly, both Chris and Hyunjin are forced to put their petty problems with the other aside - not only for the sake of their friends, but for their lives as well.
https://archiveofourown.org/works/28665117/chapters/70270422