Katya looked relieved that they were finally taking action, while Dunsany belted on his sword with a look of satisfaction, almost as though he had felt the embrace of an old friend.

“We’ll find them,” Silus said to Katya. “I know that they’re alive.”

“Really. Is this prescience another one of your powers?”

Silus said nothing, already aware of just how dreadful the consequences would be for him and Katya if they didn’t find their son.

Shalim and Rosalind joined them as they set off, as did Illiun and a handful of others, including one of the sentinels. They called out to the children as they went, although the night seemed to swallow their voices almost as soon as they were out of their mouths. The desert was still and the light of the campfire faded quickly behind them. The going became harder as the slopes of the dunes became more pronounced. They would struggle uphill, ankle-deep in sand, only to reach the crest of the hill and find themselves struggling to keep upright as they half-tumbled down the other side. After mere minutes of this, Silus’s ankles were singing with pain.

“Surely they can’t have gone far?” he said to Katya. “I mean, why would they even have wandered away from camp? What is there to see out here?”

“Perhaps they were taken,” Katya said.

“Don’t say that. Please. We must hope for the best.”

They crested the next rise, to see a scattering of huge stones protruding from the sand: boulders, scoured smooth by the desert winds, their surfaces so polished that even in the dim starlight Silus could see his reflection in them. They looked like great black jewels. He wondered whether this was what the desert itself was made from, these huge rocks whittled down to grains by the passage of time and the weather.

Ahead of them, the sentinel had stopped before a cluster of the dark rocks. He held his metal staff ahead of him, his head cocked to one side, as though listening to something. Then he placed his staff on the ground and began weaving around the rocks. At first he appeared to be randomly careening amongst the boulders, but as Silus watched, he realised the sentinel was walking around the same seven stones, in a wide double loop.