In Despair by Megan Derr

Prince Telmé Guldbrandsen has been groomed since childhood to become a Prince of the Blood and Commander of the Legion. He will be the youngest person to ever take the Blooding—if he can behave long enough to prove he can be trusted with the responsibility. But behaving is difficult when he is constantly forced to endure Korin: heir to the Reach of the House and the Temple of the Sacred Three, and the snotty brat Telmé is expected to someday marry. Then the unthinkable happens, leaving Castle Guldbrandsen—and the Legion—in pieces. Overwhelmed by fear and grief, Telmé convinces Korin to help him attempt the impossible. But rather than relief, Telmé's triumph is met with anger and rejection...