King Breaker by Rowena Cory Daniells

THE CONCLUSION TO THE HUGELY POPULAR KING ROLEN'S KIN SERIES!
When Cobalt stole the Rolencian throne, Byren, Fyn and Piro were lucky to escape with their lives; now, they've rallied, and set out to avenge their parents' murder.
Byren is driven to defeat Cobalt and reclaim the crown, but at what cost? Fyn has sworn to serve Byren's interests, but his loyalty is tested when he realises he loves Byren's betrothed. And Piro, who never wanted to win a throne, now holds the fate of a people in her hands.