The Nephilim’s amused condescension was clear even before Zan translated. ‘Some tools of the Nephilim can only be used by priestesses, but a fortress that even the ruler could not enter would be useless.’

‘Then tell him to open it,’ said Colonel Wu firmly as Nina frowned. ‘Now.’

Zan passed on the command. The giant glowered at the officer, but climbed down into the trench. The soldiers backed away – though at another order, they brought up their weapons to cover him.

Gadreel eyed them, then spoke, Zan acting as relay. ‘He said, “Do you not trust me?”’ Hui told Nina. ‘“It is just a door.”’

‘A door that only he knows what’s behind,’ she pointed out.

‘Do you not trust him?’ asked Hui.

Her reply was sardonic. ‘My trust supply’s pretty depleted right now.’ She lowered her voice. ‘He lied.’

‘What do you mean?’

‘At the base, he said that only a priestess could open it. He wanted me here for something.’

The older woman shifted uncomfortably. ‘Colonel Wu would have made you come here no matter what.’

‘Why? What’s he got in mind for me?’

Hui was reluctant to answer. She looked around in relief as father and daughter concluded a muted discussion of their own. The major told the soldiers to lower their weapons. Gadreel waited for them to retreat, then placed his palm against the disc.

Nothing happened. Colonel Wu’s face darkened, and even Gadreel appeared concerned – then the metal underfoot shuddered with a deep, heavy thump. Another sound, the grind of a mechanism moving for the first time in untold centuries, and the door began to open.

A gust of escaping air kicked up a whirl of sand around Gadreel’s feet. The fortress had been tightly sealed. He stepped back as the oval hatch swung inwards.

Colonel Wu entered the trench, standing beside – and just ahead of – Gadreel as the interior was revealed. A passageway disappeared into darkness. Hui began excited commentary into her headset for the benefit of those at the base.

Nina moved for a better look. Major Wu was about to wave her back when her father interceded. ‘Come, look,’ he said, gesturing at the opening. ‘You went in other fortress. You know what inside.’ A meaningful look at the Nephilim. ‘You tell us if he try tricks.’

She watched as the door jolted to a stop, fully open. ‘So . . . who’s going in first?’

‘You,’ said the major. ‘As long as your husband and daughter are our guests,’ a thin smile, ‘we trust you.’

Colonel Wu called out to his men, several hurrying back to the helicopters. They soon returned bearing assorted equipment, including hand-held and clip-on flashlights – and the qi tracker’s crate. ‘Why are you bringing that?’ Nina demanded.

The colonel ignored her, staring into the shadows beyond the hatch. ‘Professor Wilde,’ he said. ‘Lead the way.’

Nina sighed, then took a flashlight from Hui. ‘At least the floor’s level this time.’