Air, humid and thick, coated the inside of Samuel's mouth with an oily film, sour with the taste of effluence. Sweating, leg muscles burning, he gulped lungful after lungful of the fetid atmosphere, following the directions of the spirit compass in his hand. In the dim light of wall-mounted glow-lamps, he ran as fast as he dared along the slippery walkways. Beside him, a river of rancid sewage water flowed. Behind him, Macy and Bryant, his fellow agents of the Relic Guild, kept up easily. With their endless reserves of strength, the twins were snapping at Samuel's heels, egging him on, pushing him to increase his speed as he led the group further into the sewers beneath the streets of Labrys Town.

According to the spirit compass, the automaton spider was moving steadily towards the eastern district. Invisible to the naked eye, the metal construct, with its melon-sized body, long, spindly legs and the head of a golem, had been climbing and jumping and scurrying its way across town ever since Hamir had released it from the warehouse in the southern district. And the Relic Guild had been shadowing its path from below.

Hamir had warned the group that the spider would move fast, relentless in its pursuit of Fabian Moor, and the agents would have a hard time keeping up with it. Samuel could not allow himself a moment's respite in which to catch his breath. All he could do was keep running and focus on the spirit compass as it tracked the magic of the Genii preserved in the spider's golem-head.

Attempting a sharp left off the walkway and into a tunnel, Samuel lost his footing on the slimy stone. He began skidding and stumbling towards the river of sewage. Just as it seemed inevitable that he would fall in, the strong hand of Macy gripped the back of his coat. She flung him into the tunnel, and Samuel ricocheted off the wall. Macy kept him upright and shoved him back into a run.

'Look lively, Samuel,' she called, clearly amused. 'There's a Genii to catch.'

Bryant laughed along with his sister.

The twins were enjoying the thrill of the hunt, but Samuel wished there was time to give them a splash of cold water. Their confidence should have bolstered his own, but for all their magical strength, Macy and Bryant did not carry the burden of knowledge Samuel did; they had yet to see what Fabian Moor's virus did to a person. They seemed to have forgotten that this mission was not only to hunt and capture a Genii, but also to rescue a fellow agent of the Relic Guild.

It had been more than a day since Fabian Moor had abducted a magical apothecary called Gene. An elderly agent, quiet in his ways, Gene had never been blessed with courage. Fabian Moor wanted to know the identities of the Relic Guild agents; he wanted to extract secrets from them that would show him how to enter the Nightshade. Gene certainly didn't have the defences to stop Moor taking what he wanted. He wouldn't last long under the interrogation of a Genii. Would any of them? Samuel prayed to the Timewatcher that the automaton spider did its work quickly enough to save Gene's life. And perhaps the lives of all of them.

At the end of the tunnel, he led the twins across a bridge that spanned the river, down another tunnel, and then along the walkway on the other side.

Samuel wore a pair of goggles that Hamir had given him. They were heavy, stifling. The faceted, green glass lenses looked like the eyes of an insect, and were imbued with magic. They had turned Samuel's world to shades of grey – not that there was much colour to see down in the sewers. Hamir had designed the goggles to see through illusions, to penetrate the spell of invisibility that had been cast upon the automaton spider; but Samuel had to marvel at how the lenses also gave his pencil-etched world such depth and detail.

The mortar between brickwork, the glow lamps fixed to the walls, the rough texture of the walkway, the mammoth stone rafters high above – everything came to Samuel in its own unique shade of grey. But the goggles made the way through the sewers clear and stark, all dimness and shadow banished. Clear though the route was, the trek was pushing Samuel to the limits of his endurance.

After an hour of hard running, the spirit compass became confused, its needle spinning undecidedly around its face, and it was with immense relief that Samuel drew the group to a halt. He bent double – one hand clutching the compass, the other on his knee – and gulped down breaths of bitter air.

Macy walked up to him. Tall and broad, her blonde, shoulder-length hair pulled back into a tail, there was a frown on her not-quite masculine face. Bryant stood next to her. His hair was the same colour as his sister's, but close-cropped; his face was almost identical – apart from a scar that slashed down his cheek from the corner of his left eye.

There was not one bead of sweat on the brow of either siblings, and they were barely out of breath. At that moment, Samuel considered their magical gift of strength and fitness much preferable to his prescient awareness.

'Why have we stopped?' Macy asked.

'The Spider,' Samuel said between breaths. 'We must be ... standing directly ... below it.'

She looked up at the sewer ceiling, high above. 'It's found Moor?'

'Likely ... but its path ... might be blocked.'

'How can we tell?'

'Give it a minute.' Samuel took a huge breath, and showed Macy the spinning needle on the compass. 'Let's see ... if it starts moving again.'

'We must be well into the eastern district by now,' Bryant said. He looked around. 'I'll go and find a way out, just in case.' And he set off along the walkway.

Samuel stood upright, shifted the goggles onto his forehead, and wiped sweat from his eyes. The gloom of the sewers returned. He thought about removing his coat, but he didn't suppose the humid atmosphere would be any more forgiving on his damp skin.

Macy raised an eyebrow at him. 'Are you all right?'

Samuel nodded and steadied his breathing.

Like her brother, Macy wore a thick cloak over her clothes, the hood of which was made from charmed material that would steep her identity in shadows. She also wore gauntlets and a gorget made of black leather stuffed with chain mail. When Fabian Moor's hiding place was discovered, the three agents would likely be facing victims of the Genii's virus – a virus that drove a person mad with a savage thirst for blood before eventually converting all their organic matter into animated stone, and creating a simple-minded golem. The twins had taken every precaution against getting bitten and infected, but only on Samuel's insistence.

'I've found the way out,' Bryant called from a little way down the walkway.

Samuel checked the spirit compass – the needle still spun around the face. He nodded to Macy.

She gave a crooked smile. 'Your power stones are charged, right?'

Samuel pulled the goggles down over his eyes. 'Let's go.'

Bryant led the way up a caged ladder fixed to the sewer wall to a metal grille platform and a set of narrow stairs ascending to a hatchway in the sewer ceiling. Samuel put on his wide-brimmed hat made from charmed Aelfirian material, and his face became hidden in shadows. Macy and Bryant hitched up the hoods of their cloaks, and the three agents climbed out into fresher air and the clear morning sky over the eastern district.

The alleyway they entered was deserted; it was narrow and the walls were high. The sun didn't shine in the alley, and the shadows retained the chill of night. The hatchway was imbued with magic and became indistinguishable from the cobbled ground when Bryant closed it. The spirit compass' needle fixed onto a direction and led Samuel to the end of the alley. He smelled wood smoke and drew the twins to a halt.

Peeking around the corner, Samuel saw two vagrants – a man and a woman – sitting in an overgrown communal garden before a small fire made from broken packing crates. On the other side of the garden, a shabby apartment block rose three storeys. It was a squat and ugly building, rickety, somehow looking like a crooked old man. Through the lenses of the goggles, the grey shades of its brickwork seemed scorched and pockmarked, its windows cracked and thickly grimed. And halfway up, the automaton spider clung to its face.

Though the spider was invisible to the naked eye, the magic in the goggles revealed it as a huge eight pointed star shape made from wispy, purple smoke. It wasn't moving.

Hamir had said that if Fabian Moor detected the spider closing in on him, he could destroy it easily. The necromancer needed Samuel and the twins to divert Moor's attention – which was a polite way of saying the agents needed to be bait – and keep the Genii unaware of the spider's mission. Did the arachnid construct understand the situation? It had been created from a thaumaturgic metal that was supposedly sentient. Did that mean it could know to wait for the Relic Guild's diversion?

'What's going on?' Bryant asked.

'I can see the spider,' Samuel said.

'What's it doing?'

'Waiting for us, I think. I hope.'

'I know that place,' Macy said, indicating the building. 'They call it a shelter.' There was a bitter edge to her voice. 'But really, it's where the infirm and homeless get dumped and forgotten.'

'Perfect place for Moor to hide,' Samuel said. He slipped the spirit compass into his coat pocket. 'Don't forget what I told you,' he warned the twins.

The three agents stepped from the alley and made their way across the garden. The vagrants ceased their conversation and stared at them. The old woman began cackling as they neared; the man grinned, revealing crooked and stained teeth.

'Well, look at this,' he said, gesturing to the hidden faces. 'A visit from the Resident's men.'

'And woman,' said the woman. She seemed pleased with herself and jabbed a finger at the man beside her. 'Told you something strange was going on. Didn't I tell you?'

Macy was the first to reach the pair. She crouched before the fire, slipped off her gauntlets, and warmed her hands while Samuel and Bryant stood watching.

'And what are you two doing out here?' she said genially. 'Why aren't you inside that place?'

'It's not from lack of trying,' the old woman said, looking at the shelter building behind her. 'The doors are locked, and no one's gone in or out for two days.'

'You're right,' said Macy, 'that is strange.'

'But don't think that's the end of it,' the old man chipped in. He too glanced at the building, and he shivered. 'The noises that come out of that place ain't right. Sounds like someone's in a bad way.'

'More than just someone, I reckon,' the woman added. She leant forward and gave Macy a gummy smile. 'Let's just say we're not surprised the Resident sent the Relic Guild, dear.'

The man agreed with a wide-eyed nod.

Macy looked quickly at Samuel and her brother, then turned back to the vagrants. 'I'll tell you what,' she said, reaching under her cloak to fish out a folded wad of money. 'While we go and do our job, why don't you two catch a tram to another part of town?' She flicked off forty Labyrinth pounds and held the notes out to the woman. 'Go and find a better shelter and get some food. Sound good?'

Encouraged by a quick, affirming nod from her fellow vagrant, the old woman snatched the money from Macy's hand. 'Sounds brilliant,' she said. 'Thanks very much, dear.'

'My pleasure.'

They got to their feet and shuffled away from the fire. By the sly smiles on their faces, Samuel knew the last thing they would spend Macy's money on was food and a tram ticket. But at least it got them out of the way.

As Macy slipped her gauntlets back on, Samuel drew his revolver and thumbed the power stone set behind the chamber. It gave a brief whine and glowed with violet light. Bryant led the way to the entrance of the crooked and decaying shelter building.

Samuel crossed the garden and passed through a patch of warm sunlight. He gave an involuntary shudder when he stepped out of it and into the chill of the building's shadow. The automaton spider was still clinging motionless to the wall. Samuel followed the twins up the steps to entrance doors which were firmly locked. Through a slim gap, a rusty chain could be seen looped around the inside door handles.

'I've got a phial of acid in my belt,' Samuel said.

Bryant scoffed at his idea, leant back, and kicked the doors. They burst inwards with the ping of snapping metal.

Bryant shrugged at Samuel's irritated look. 'Hamir said to distract Moor. We might as well make some noise.'

Macy clapped Samuel's shoulder and pushed him after her twin into the building. 'Bet you wish you'd gone with Van Bam and Angel, don't you?' she chuckled.