Blood King Part 1 (Crowns #4) by Nicola Tyche:

He doesn’t want the throne. He wants revenge. A new crown. An old wound. And a reckoning written in blood. A kingdom built on blood… Raised a fighter in Rael’s brutal bloodsport arena, Cyrus has only ever known survival. Until one act of defiance topples a tyrant, and one vow of vengeance sparks a revolution. Now, he’s the man a broken realm looks to for hope. A power he can’t control… Haunted by his past and driven by revenge, Cyrus grapples with a power that has haunted him since he was a child—a power he doesn’t understand. When a sharp-tongued witch offers to help, her motives are unclear. Trust is dangerous, and so is she. A throne he didn’t ask for… Crowned in chaos, Cyrus is thrust into a world of politics, power, and war. New enemies rise. Old ghosts return. And while vengeance may have earned him a crown… It won’t be enough to keep it.