Meanwhile, in a frosted turret in Winterfang Palace, the Ice Queen sat on a throne built from walrus tusks. They jutted above her head and shoulders, a fan of pointed ivory, and beside her sat a wolverine, purring darkly. She let her fingers glide along the staff across her lap, then she raised her head to the other person in the room.

‘You have done well, Slither.’

The shaman’s face twisted into an ugly smile as he eyed the statue in the middle of the turret. It was a life-size model of a child – the eyes, nose, hair and limbs all chiselled out of glass – and, though Eska was miles and miles away, to the Ice Queen and Slither it felt as if she was standing right there on the floor in front of them.

‘I believe the contraption is working as we wanted it to,’ Slither replied. ‘There are signs on the glass to prove it.’

He pointed to the spidery lines spreading out from the hollow in the statue’s throat and the Ice Queen’s blue lips quivered as she took them in. Then she stood up and stalked towards the statue, raking her staff across the frosty tiles. She held her crown of snowflakes high, then lifted the black key from the chain around her neck and slotted it into the hole in the statue’s throat.

Her eyes flickered towards Slither. ‘The magic in this turret only works when I play the organ – it is fuelled by my growing strength – so I shall play the organ in the evenings as well as the mornings from now on.’ She paced towards the window and looked out at the stars. ‘Because we only have five days before the midnight sun rises.’

Slither’s scalp tightened. ‘We must be careful not to move too quickly. Magic used in haste often crumbles. Perhaps we could—’

The Ice Queen cut across him. ‘I need Eska’s voice drained from her body before she realises she can claim the Sky Song for her own. So, when you hear my choir sing, I want you to turn and turn and turn.’

She left the room, the wolverine by her side, and, as the organ sounded a moment later and the prisoners’ voices sailed into the night, Slither wound the key again and again and more dark veins seeped across the statue, scarring it through with black.