It was quiet inside the monastery and the shrieks of the fighting were muffled by the thick stone walls that lined the passageway in front of them. Moll tiptoed down it with Gryff, the Oracle Arrow nocked loosely to her bow. Wax drooled from candles in iron brackets either side of them and Moll flinched at the clacking of her boots against the paving stones. Somewhere, in the shadows of this place, Wormhook was waiting for her. She crept on, and with every step she took she tried to draw courage from the wildcat padding faithfully by her side.

After a few minutes, they came to an arch made entirely of thorns. Perhaps once, in the times when monks had used the place as a sanctuary for prayer, there had been roses here, but now dark thorns twisted above them like a mouth of tangled wire.

Moll swallowed. ‘We can do this.’

Gryff nuzzled her hand with his head, then together they stepped through the arch. But, whatever Moll had been expecting on the other side, it wasn’t what lay before her. An enormous cavern carved into the rock stretched fifty metres above and below them. A large stone bridge ran through the middle of the cavern, its supporting pillars entwined with dead ivy and lichen, and as Moll took a step out on to it dozens of black feathered heads poked out from the nests perched on the crags of the walls. They croaked into the silence, knowing there was an intruder in their midst. Moll glanced over the edge of the bridge to see the hall at the bottom was lined with statues of monks, the stonework smeared with bird droppings, but Gryff tugged at Moll’s coat sleeve with his teeth and urged her on.

Where the bridge met the far side of the cavern there was another arch of thorns. Moll and Gryff stole through it to find a lantern inside flickering over three passageways. Those to her left and right were draped in shadows – stairs, it looked like – perhaps up to the turrets they had seen when they first laid eyes on the Rookery with Wallop. But Moll knew it was the passageway ahead that she wanted because there was a path of light coming from it and the rustle of paper being turned. Gryff blinked up at Moll and they walked on.

The thorns opened into a large room and Moll blinked in disbelief as she took it in. Lining the stone walls were trees. Crooked trunks twisted upwards and thin grey branches, from which lanterns dangled, sprawled across the slabs of stone. A long time ago, they might have been rowans bursting with berries – the last of the trees to survive the mountains and the cold – but now bark flaked from their trunks, dead leaves lay scattered around their bases and twisted roots scarred the floor. It was like being in a pocket of the forest, where all of this had started. But along the branches that stretched across the walls, in among the lanterns, there were books – beautiful leather spines with gilt lettering – laid out as if the branches themselves were shelves. This was a library and standing behind a lectern in the middle of the room, facing them, was Wormhook.

An enormous leather-bound book, almost the size of Moll, with edges dusted gold, rested on the lectern in front of the last Shadowmask and as Moll entered he glanced up, a black quill feather poised in his hand.

‘The Ancient Book that Wallop spoke about,’ Moll said slowly, raising her bow, ready to fight. Gryff growled by her side and Moll felt her voice harden. ‘What have you done to it?’

Wormhook heaved the book shut and a cloud of dust puffed upwards. ‘Oh, you’ll find out soon enough.’

Gryff sprang forward, but the Shadowmask lurched back from the lectern, his robes a ripple of black, and from the slit in his sack mask, where his mouth should have been, something dark and swirling seeped out. It floated into the library like a trail of ink.

‘A Night Spinner’s gifts extend beyond conjuring storms and snow,’ Wormhook sneered. ‘I can call upon nightmares too – the darkest thoughts from the Underworld. They breathe fear and they have the power to tear you and your wildcat apart.’

The ink spilled into a large shadow and the unmistakable shape of a wolf hung before Gryff, its body and head bent low, its ears pinned back. The wildcat edged away, hissing, but the shadow grew in size, its monstrous head thrashing from side to side. And then the wolf pounced.

‘It’s not real!’ Moll yelled to Gryff. ‘It’s just a nightmare!’

Wormhook’s sack mask tilted, as if half amused by Moll’s words. ‘Not real?’ he muttered. ‘Oh, it’s real all right.’

Moll watched in horror as the shadow hurled Gryff across the room, smashing him into the trunk of a tree. The nightmare had a mind of its own and, as Moll got ready to fire her arrow, more ink poured from Wormhook’s mouth. Moll strained her eyes through the pitch-blackness, searching for the witch doctor, but he was hidden now and the nightmares slid closer, morphing into the shapes of giant bats. Moll felt a coldness cling to her skin. The darkness was absolute and, as the swarm of bats opened their wings over her face, she felt her bow and arrow clatter to the ground.

‘They’re not real!’ Moll whimpered. ‘They’re not real!’

She struck her arms out against the shadows and for a second the darkness broke apart and she saw Gryff backed up against the lectern, thrashing his paws towards the wolf. Then the nightmares closed round her again, pulsing into a throng of bats, and, though they looked to be only shadows, claws raked at her face and leathery wings smacked her body. Moll fell to her knees as the nightmares sucked at her soul.

‘Your parents were too weak to fight us and they died as cowards, snivelling for the child they’d left behind,’ Wormhook taunted. ‘We expected more from the child and the beast, but look at you both now.’

The bats clamoured around Moll, scrabbling at her face, and she was dimly aware of a sack-like hand snatching at something by her feet.

‘You may have destroyed the Veil and the other five Shadowmasks,’ Wormhook jeered, ‘but I am more powerful than those you faced before and I am close to finishing your miserable little life once and for all.’ He laughed. ‘And when that is done, Molly Pecksniff, the dark magic will rise and, thanks to you, I won’t even need to share the power with the other witch doctors; it will be mine and mine alone!’

Moll’s neck throbbed as a bat tightened its giant claws around her, strangling the air in her throat, and then she screamed as she felt the skin at her nape tear. She scrabbled for the Oracle Arrow on the ground, but it was gone, taken by Wormhook, and Moll’s heart shook.

Give up, a voice inside her whispered. In the end they’ll win, whatever you do. Alfie’s gone – you can’t pull him back from what Wormhook has made him – and the others are outside fighting a losing battle. The Shadowmasks’ darkness is too strong; you never really stood a chance.

The voices inside Moll grew and her shoulders slumped. They’d made it this far – to the monastery tucked into the clouds – but Wormhook’s nightmares were a darkness too strong for any soul to withstand. Moll wrapped her arms round her head and let the despair crawl all over her as the last Shadowmask made his way back to the lectern, the Oracle Arrow in one hand, his quill held high in the other.