After satisfying himself that the harnesses were secure, Thomas turned and ran lightly up the steps, stopping when Tessa raised a hand to halt him. “Are they going now?” she asked. “Is that all?”

He nodded. “All ready to go, miss.” He had tried to get Jem and Will to take him along, but Will was concerned that Charlotte would be angry at Thomas for participating in their exploit and had told him not to come.

“Besides,” Will had said, “we ought to have a man in the house—someone to protect the Institute while we’re gone. Nathaniel doesn’t count,” he’d added, with a sideways glance at Tessa, who had ignored him.

Will slid Jem’s sleeve down, covering the Marks he had made. As he returned his stele to his pocket, Jem stood looking up at him; their faces were pale smudges in the torchlight. Tessa raised her hand, then lowered it slowly. What was it he had said? Shadowhunters don’t say good-bye, not before a battle. Or good luck. You must behave as if return is certain, not a matter of chance.

The boys, as if alerted by her gesture, looked up toward her. She thought she could see the blue of Will’s eyes, even from where she stood. He wore a strange look as their eyes locked, the look of someone who has just woken up and wonders if what they are looking at is real or a dream.

It was Jem who broke away and ran up the stairs to her. As he reached her, she saw that he had high color in his face, and his eyes were bright and hot. She wondered how much of the drug Will had let him take, so that he would be ready to fight.

“Tessa—,” he said.

“I didn’t mean to say good-bye,” she said quickly. “But—it seems odd to let you leave without saying anything at all.”

He looked at her curiously. He did something that surprised her then, and took her hand, turning it over. She looked down at it, at her bitten fingernails, the still-healing scratches along the backs of her fingers.

He kissed the back of it, just a light touch of his mouth, and his hair—as soft and light as silk—brushed her wrist as he lowered his head. She felt a shock go through her, strong enough to startle her, and she stood speechless as he straightened, his mouth curving into a smile.

“Mizpah,” he said.

She blinked at him, a little dazed. “What?”

“A sort of good-bye without saying good-bye,” he said. “It is a reference to a passage in the Bible. ‘And Mizpah, for he said, the Lord watch between me and thee when we are absent one from another.’”

There was no chance for Tessa to say anything in response, for he had turned and run down the steps to join Will, who was as motionless as a statue, his face upturned, at the foot of the steps. His hands, sheathed in black gloves, were in fists at his sides, Tessa thought. But perhaps it was a trick of the light, for when Jem reached him and touched him on the shoulder, he turned with a laugh, and without another look at Tessa, he swung himself up into the driver’s seat, Jem following him. He cracked the whip, and the carriage rattled through the gate, which slammed shut behind it as if pushed by invisible hands. Tessa heard the lock catch, a hard click in the silence, and then the sound of church bells ringing somewhere in the city.

Sophie and Agatha were waiting in the entryway for Tessa when she came back inside; Agatha was saying something to Sophie, but Sophie didn’t appear to be listening. She looked over at Tessa as she came in, and something about the way she looked, for a moment, reminded Tessa of the way Will had looked at her in the courtyard. But that was ridiculous; there were no two people in the world more unalike than Sophie and Will.

Tessa stepped aside as Agatha went to close the great, heavy double doors. She had just pushed them shut, panting slightly, when the knob of the leftmost door, untouched, began to turn.

Sophie frowned. “They can’t be back so soon, can they?”

Agatha gazed down, perplexed, at the turning knob, her hands still braced against the door—then stood back as the doors swung wide before her.

A figure stood on the doorstep, backlit by the light outside. For a moment all Tessa could tell was that he was tall and clad in a frayed jacket. Agatha, her head tipping back as she gazed upward, said in a startled voice, “Oh, my Lor’—”

The figure moved. Light flashed on metal; Agatha screamed and staggered. She seemed to be trying to back away from the stranger, but something was preventing her.

“Dear God in Heaven,” Sophie whispered. “What is that?”

For a moment Tessa saw the whole scene frozen, as if it were a painting—the open door, the clockwork automaton, the one with the stripped bare hands, still in the same worn gray jacket. And still, dear God, with Jem’s blood on its hands, dried red-black on the dull gray flesh, and the strips of copper showing through where the skin had been scraped or pulled away. One bloodstained hand gripped Agatha’s wrist; clamped in the other was a long, thin knife. Tessa moved forward, but it was already too late. The creature swung the blade with blinding speed, and buried it in Agatha’s chest.

Agatha choked, her hands going to the blade. The creature stood, ragged and terrifying and unmoving as she clawed at the knife hilt; then, with appalling swiftness, it yanked the blade back, letting her crumple to the ground. Nor did the automaton remain to watch her fall, but turned and walked back out the door through which it had come.

Galvanized, Sophie screamed “Agatha!” and ran to her side. Tessa dashed to the door. The clockwork creature was walking down the steps, into the empty courtyard. She stared after it. What on earth had it come for, and why was it leaving now? But there was no time to dwell on that. She reached for the rope of the summoning bell and pulled it, hard. As the sound clanged through the building, she slammed the door shut, dropping the lock bar into place, then turned to help Sophie.

Together they managed to lift Agatha and half-carry, half-drag her across the room, where they fell to their knees beside her. Sophie, ripping strips of fabric from her white apron and pressing them over Agatha’s wound, said in a tone of wild panic, “I don’t understand, miss. Nothing should be able to touch that door—none but one with Shadowhunter blood should be able to turn the doorknob.”

But he had Shadowhunter blood, Tessa thought with a sudden horror. Jem’s blood, staining its metal hands like paint. Could that be why it had bent over Jem that night after the bridge? Could that be why it had fled, once it had gotten what it wanted—his blood? And didn’t that mean it could come back whenever it wanted?

She began to rise to her feet, but it was already too late. The bar that held the door closed cracked with a noise like a gunshot, and tumbled to the ground in two pieces. Sophie looked up and screamed again, though she didn’t move away from Agatha as the door burst open, a window onto the night.

The steps of the Institute were no longer empty; they were teeming, but not with people. Clockwork monsters swarmed up them, their movements jerky, their faces blank and staring. They were not quite like the ones Tessa had seen before. Some looked as though they had been put together so hastily that they had no faces at all, just smooth ovals of metal patched here and there with uneven bits of human skin. Even more horrible, quite a few of them had bits of machinery in place of arms or legs. One automaton had a scythe where his arm ought to have been; another sported a saw that stuck out of the hanging sleeve of his shirt like a parody of a real arm.

Tessa rose and flung herself against the open door, trying to heave it shut. It was heavy, and seemed to move agonizingly slowly. Behind her, Sophie was screaming, helplessly, over and over; Agatha was horribly silent. With a ragged gasp Tessa pushed at the door one more time—

And jerked her hands back as the door was torn out of her grasp, ripped from its hinges like a handful of weeds ripped out of the earth. She fell back as the automaton who had seized the door flung it aside and heaved itself forward, its metal feet clanking against the stone as it lurched over the threshold—followed by another and then another of its mechanical brethren, at least a dozen of them, advancing toward Tessa with their monstrous arms outstretched.

By the time Will and Jem reached the mansion in Highgate, the moon had begun to rise. Highgate was on a hill in the north part of London, commanding an excellent view of the city below, pale under the moon’s light, which turned the fog and coal smoke that hung over the city into a silvery cloud. A dream city, Will thought, floating in the air. A bit of poetry hung at the edges of his mind, something about the terrible wonder of London, but his nerves were tight with the jangling tension of impending battle, and he could not remember the words.

The house was a great Georgian pile, set in abundant parkland. A high brick wall ran around it, the slanting dark mansard roof just visible above it from the street. A shiver of cold passed over Will as they drew near it, but he was unsurprised to feel such a thing in Highgate. They were near what Londoners called the Gravel Pit Woods at the city’s edge, where thousands of bodies had been dumped during the Great Plague. Lacking a proper burial, their angry shades haunted the neighborhood even now, and Will had been sent up here more than once, thanks to their activities.

A black metal gate set into the mansion’s wall kept out intruders, but Jem’s Open rune made short work of the lock. After leaving the carriage just inside the gate, the two Shadowhunters found themselves on the curving drive that led up to the house’s front entrance. The path was weed-ridden and overgrown, and the gardens stretched out around it, dotted with crumbling outbuildings and the blackened stumps of dead trees.

Jem turned to Will, eyes feverish. “Shall we get on with it?”

Will drew a seraph blade from his belt. “Israfel,” he whispered, and the weapon blazed up like a fork of contained lightning. Seraph blades burned so brightly that Will always expected them to give off heat, but their blades were ice cold to the touch. He remembered Tessa telling him that Hell was cold, and he fought back the odd urge to smile at the memory. They’d been running for their lives, she ought to have been terrified, and there she had been, telling him about the Inferno in precise American tones.

“Indeed,” he said to Jem. “It’s time.”

They ascended the front steps and tried the doors. Though Will had expected them to be locked, they were open, and gave way at the touch with a resonant creaking. He and Jem edged inside the house, the light of their seraph blades illuminating the way.

They found themselves in a grand foyer. The arched windows behind them had probably once been magnificent. Now they alternated whole panes with broken ones. Through the spiderwebbing cracks in the glass, a view of the tangled and overgrown parkland beyond was visible. The marble underfoot was cracked and broken, weeds growing up through it as they had been growing through the stones of the drive. Before Will and Jem, a great curving staircase swept upward, toward the shadowy first floor.

“This can’t be right,” Jem whispered. “It’s as if no one’s been here in fifty years.”

Barely had he finished speaking when a sound rose on the night air, a sound that lifted the hairs on Will’s neck and made the Marks on his shoulders burn. It was singing—but not pleasant singing. It was a voice capable of reaching notes no human voice could reach. Overhead, the chandelier’s crystal pendants rattled like wineglasses set to vibrating at the touch of a finger.