So a prince dies after making a truce with another empire and their crown prince Jiskar. However, on the way home he dies and in reincarnated as a slave in the neighboring kingdoms empire. Things get interesting however when he meets Jiskar again and calls out the emperor's name (apparently that's a no no). So naturally this slave who remembers being a prince tries to gain more freedom while being the emperor's concubine.
The politics are fun
Two Empires, The Slave Of The Empire