“Then we better find the other two before they harm our children,” Rosalind said.

They followed Rosalind and Shalim past the corpse of the sentinel and further out into the desert.

It wasn’t long before they heard Zac and Hannah crying.

“Oh, thank the gods,” Katya said, running towards the sound.

They came to a depression ringed by more of the black stones, arranged so as to form a natural amphitheatre, at the centre of which was a bizarre, chilling tableau.

One of the sentinels was crouched over Zac with his right knee resting in the middle of the child’s back, pinning him to the ground. Blue froth slowly bubbled from the sentinel’s mouth as he gathered small stones towards himself, dragging them through the sand and carefully arranging them in two neat rows by Zac’s head. Occasionally the sentinel would twitch violently, emitting a noise like sheet metal tearing; eliciting, in turn, even greater cries from the boy.

At first they couldn’t see Hannah, only the form of the second sentinel as he capered around the perimeter of the depression on all fours, throwing up screeds of sand as he suddenly changed direction. But then they all heard the child’s cry and saw Hannah dart between two huge, jagged rocks. She was bleeding from a graze on her forehead and, as they watched in horror, the sentinel raced towards her, lashing out with his metal staff, striking the rock just above her head.

Shalim was already sprinting towards the sentinel, but the silver-eyed man was considerably swifter and soon lost him amongst the stones. Hannah threw herself into her father’s arms, sobbing and looking towards where her friend was pinned to the sand.

“Help Zac, Daddy, please.”

But Shalim was too concerned with getting his daughter out from amongst the stones to be able to help Zac. Besides, both Dunsany and Silus were already racing towards the boy.