What is A Reaper at the Gates (Ember Quartet, Book 3) by Sabaa Tahir about?

Beyond the Empire and within it, the threat of war looms ever larger.

The Blood Shrike, Helene Aquilla, is assailed on all sides. Emperor Marcus, haunted by his past, grows increasingly unstable, while the Commandant capitalizes on his madness to bolster her own power. As Helene searches for a way to hold back the approaching darkness, her sister’s life and the lives of all those in the Empire hang in the balance.

Far to the east, Laia of Serra knows the fate of the world lies not in the machinations of the Martial court, but in stopping the Nightbringer. But while hunting for a way to bring him down, Laia faces unexpected threats from those she hoped would aid her, and is drawn into a battle she never thought she’d have to fight.

And in the land between the living and the dead, Elias Veturius has given up his freedom to serve as S