A Torch Against the Night (An Ember in the Ashes #2) By Sabaa Tahir

After the events of the Fourth Trial, Martial soldiers hunt Elias and Laia as they flee the city of Serra.

Laia and Elias are determined to break into the Empire's most secure and dangerous prison to save Laia's brother, even if for Elias it means giving up his last chance at freedom.

They will have to fight every step of the way to outsmart their enemies: the bloodthirsty Emperor Marcus, the merciless Commandant, the sadistic Warden of Kauf, and, most heartbreaking of all, Helene - Elias's former friend and the Empire's newest Blood Shrike.

Bound to Marcus's will, Helene faces a torturous mission of her own - one that might destroy her: find the traitor Elias Veturius and the Scholar slave who helped him escape . . . and kill them both.