Kingdom of Ash by Sarah J. Maas

Aelin Galathynius has vowed to save her people, but at a tremendous cost. Locked in an iron coffin by the Queen of the Fae, Aelin must draw upon her fiery will as she endures months of torture. The knowledge that yielding to Maeve will doom those she loves keeps her from breaking, but her resolve unravels with each passing day. 

With Aelin captured, her friends and allies have scattered. Some bonds grow even deeper, while others will be severed forever. But as destinies weave together at last, all must stand together if Erilea is to have any hope of salvation. 

Sarah J. Maas's #1 bestselling Throne of Glass series draws to an explosive conclusion as Aelin fights for her life, her people, and the promise of a better world.Other books in this series include: 

Throne of Glass
Crown of Midnight
Queen of Shadows
Empire of Storms
Tower of Dawn
Kingdom of Ash
The Assassin's Blade (prequel novellas)