Black Knight Series by Kai Tsurugi

Zeke O'Brien may have his eyes on the title of Black Knight--the highest honor that can be bestowed upon a knight in the kingdom of Aran--but he's also after another prize: The young prince, Chris! Black Knight is a sweeping romantic epic about the relationship between the prince of a medieval kingdom and his loyal guardsman, as they embark on a life-long relationship fraught with danger, treachery, and above all, love.

Eager to prove his worth to his kingdom, Prince Christian has taken cover in Brigadoon's caravan as a hostage to infiltrate the Dundalk Empire. Negotions for peace between the kingdoms is all the young prince yearns for, but the rebellious Falcon's Talon group has something else in mind...Now Zeke's loyalty as Christian's knight and lover will be put to the test in this heroic quest for justice!