House of Ruin by Miriam Newman (The Books of Scion 2)
Blurb: When the beleaguered citizens of the capitol city of Thelona are swept by plague in the absence of their ruling class, who have fled it en masse, desperation finally pushes them to take to the streets. With their city burning, the Lords of Thelona have no choice but to shoulder the burden of responsibility. In the absence of their Highest, they turn to Caius, Scion of the House of Bardin. He has unwittingly placed himself in the path of his ruler’s ambitions and also endangered his human slave, Lela. What will become of the House of Bardin?