Julian looked from Arthur to Mark to the other two faeries. “He’s in shock,” he said. “His health is fragile; it has been since the war.”

“It’s from an old Shadowhunter poem,” said Emma. “I’m surprised you don’t know it.”

“Poems contain much truth,” said Iarlath, and there was humor in his voice, but a bitter sort. Emma wondered if he was laughing at them or himself.

Julian was staring at Mark, a look on his face of unmitigated shock and longing. “Mark?” he said.

Mark looked away.

Julian looked as if he had been pierced by elf-bolts, the sly faerie arrows that burrowed under the skin and released deadly poison. Any anger Emma had felt toward him about the night before evaporated. The look on his face was like knife blades in her heart. “Mark,” he said again, and then in a half whisper, “Why? Why can’t he speak to me?”

“He is forbidden by Gwyn to speak until our bargain is sealed,” said Kieran. He glanced at Mark, and there was something cold in his expression. Hatred? Envy? Did he despise Mark for being half-human? Did they all? How had they showed their hate all these years, when Mark was at their mercy?

Emma could sense how hard Julian was holding himself back from going to his brother. She spoke for him. “So Mark is your bargaining chip.”

Rage flashed across Kieran’s face, sudden and startling. “Why must you state things that are obvious? Why must all humans do it? Foolish girl—”

Julian changed; his attention snapped away from Mark, his spine straightening, his voice hardening. He sounded calm, but Emma, who knew him so well, could hear the ice in his voice. “Emma is my parabatai,” he said. “If you ever speak to her like that again, there will be blood on the floor of the Sanctuary, and I do not care if they put me to death for it.”

Kieran’s beautiful, alien eyes gleamed. “You Nephilim are loyal to your chosen partners, I will give you that.” He waved a dismissive hand. “I suppose Mark is our bargaining chip, as you put it, but do not forget that it is the fault of the Nephilim that we need one at all. There was a time when Shadowhunters would have investigated the killings of our kind because they believed in their mandate to protect more than they believed in their hate.”

“There was a time when the Fair Folk might have freely returned to us one of our own,” said Arthur. “The pain of loss goes both ways, as does the loss of trust.”

“Well, you will have to trust us,” said Kieran. “You have no one else. Do you?”

There was a long silence. Julian’s gaze went back to his brother, and in that moment Emma hated the Fair Folk, for in holding Mark, they also held Julian’s human, breakable heart. “So you want us to find out who is responsible for these killings,” he said. “Stop the murders of faeries and humans. And in return you will give us Mark, if we succeed?”

“The Court is prepared to be far more generous,” said Kieran. “We will give you Mark now. He will assist you in your investigation. And when the investigation is over, he may choose whether he remains with you or returns to the Hunt.”

“He will choose us,” Julian said. “We are his family.”

Kieran’s eyes shone. “I would not be so sure, young Shadowhunter. Those of the Hunt are loyal to the Hunt.”

“He isn’t of the Hunt,” Emma said. “He’s a Blackthorn.”

“His mother, Lady Nerissa, was fey,” said Kieran. “And he has ridden with us, reaped the dead with us, mastered the use of elf-bolt and arrow. He is a formidable warrior in the faerie fashion, but he is not like you. He will not fight like you. He is not Nephilim.”

“Yes, he is,” said Julian. “Shadowhunter blood breeds true. His skin can bear Marks. You know the laws.”

Kieran did not reply to that, just looked at Arthur. “Only the head of the Institute can decide this. You must let your uncle speak freely.”

Emma looked to Arthur; they all did. Arthur picked nervously, fretfully at the arm of his chair. “You wish the fey boy here that he might report on us to you,” he said finally in a quavering voice. “He will be your spy.”

The fey boy. Not Mark. Emma looked over at Mark, but if a flicker of hurt passed across his stony face, it was invisible.

“If we wished to spy on you, there are easier ways,” said Kieran in a tone of cold reproach. “We would not need to give up Mark—he is one of the best fighters of the Hunt. Gwyn will miss him sorely. He will not be a spy.”

Julian drew away from Emma, fell on his knees by his uncle’s chair. He leaned in and whispered to Arthur, and Emma strained to hear what he was saying, but could make out only a few words— “brother” and “investigation” and “murder” and “medicine” and “Clave.”

Arthur held up a shaking hand, as if to silence his nephew, and turned to the faeries. “We will accept your offer,” he said. “On the condition that there will be no tricks. At the end of the investigation, when the killer is caught, Mark will make his own free choice to stay or to go.”

“Of course,” said Iarlath. “As long as the murderer is clearly identified. We wish to know the one with the blood on his hands— it will not be enough for you to say ‘it was done by that one or this one’ or ‘vampires were responsible.’ The murderer or murderers will be placed in the custody of the Courts. We will mete out justice.”