Would you trade your soul mate for your soul?

A Shadowhunter’s life is bound by duty. Constrained by honor. The word of a Shadowhunter is a solemn pledge, and no vow is more sacred than the vow that binds parabatai, warrior partners—sworn to fight together, die together, but never to fall in love.

Emma Carstairs has learned that the love she shares with her parabatai, Julian Blackthorn, isn’t just forbidden—it could destroy them both. She knows she should run from Julian. But how can she when the Blackthorns are threatened by enemies on all sides?

Their only hope is the Black Volume of the Dead, a spell book of terrible power. Everyone wants it. Only the Blackthorns can find it. Spurred on by a dark bargain with the Seelie Queen, Emma; her best friend, Cristina; and Mark and Julian Blackthorn journey into the Courts of Faerie, where glittering revels hide bloody danger and no promise can be trusted. Meanwhile, rising tension between Shadowhunters and Downworlders has produced the Cohort, an extremist group of Shadowhunters dedicated to registering Downworlders and “unsuitable” Nephilim. They’ll do anything in their power to expose Julian’s secrets and take the Los Angeles Institute for their own.

When Downworlders turn against the Clave, a new threat rises in the form of the Lord of Shadows—the Unseelie King, who sends his greatest warriors to slaughter those with Blackthorn blood and seize the Black Volume. As dangers close in, Julian devises a risky scheme that depends on the cooperation of an unpredictable enemy. But success may come with a price he and Emma cannot even imagine, one that will bring with it a reckoning of blood that could have repercussions for everyone and everything they hold dear.