The Crown Prince by Lichelle Slater

The world I knew was destroyed the day a princess showed me mercy.
Elisa should have killed me for murdering her younger sister, Dahlia. It was her right and my punishment. Justice. Instead, she let me go.
That act of mercy started me down a course I never would have chosen otherwise. Mercy filled me with doubt toward Selina’s motives, and I began to question my orders.
But it wasn’t until I met Princess Ismae that I realized I was much stronger than Selina made me believe. Foolishly, I stood up to her and tried to take the four stones and release the spell on the four kingdoms on my own. I failed.
Once again Selina’s puppet, I must face myself and learn the truth of my father in order to rid the world of Selina once and for all.
Even if it means sacrificing myself.