There was a new urgency in Selethen's manner as the outriders closed in, in response to his signal. He looked around the foreigners, ensuring they were all wearing the kheffiyehs he had given them when they set out from Al Shabah. These were desert headdresses – essentially a simple square of cotton, folded into a triangle, then draped over the head so that elongated tails hung down either side and at the back, providing protection from the sun. They were held in place by a twisted coil of camel hair rope.

Now he quickly showed them how the elongated tails could be pulled across the face then quickly twisted over each other to cover the nose and mouth of the wearer. It was a simple but effective form of head protection in the desert.

'You'll need them,' he said. 'Once the sand wall hits us, you'll be unable to breathe without them.'

Will glanced to the south. The thin dark line he had noticed a few minutes ago was now a thick band that spread from one side of the horizon to the other. In fact, he realised, the horizon seemed to have moved closer. He glanced north to confirm the fact. The sandstorm was blotting out the horizon to the south. It was a dirty brown colour at the base, almost black. And now he could see it as it towered thousands of metres into the air, blocking out the sky. The storm itself was rapidly becoming the boundary of their world.

Selethen stood in his stirrups, looking for any available shelter.

'There.' he called. 'There's a shallow wadi. The bank will give us a little protection.'

He urged his horse towards the wadi, a dry gully that jut through the hard rocky ground. The walls were barely three metres in height but they would offer some protection, at least. They hurried to follow him. He halted a few metres short of the edge to allow them to pass by.

'My god,' said Horace, 'look at how fast it's moving!'

They looked up. The dirty brown wall of swirling sand now completely blocked their sight to the south. There was nothing but the storm and now they could see how quickly it was advancing on them. It was moving like the wind, Will thought. Then he realised, it was the wind.

He glanced up and caught Evanlyn's eyes on him. They exchanged a worried look and he knew they were both thinking of the same thing – the massive storm that had swept down on them when they were prisoners on Wolfwind years before. He tried to grin reassuringly at her but at that moment the first breath of the storm struck them – unbelievably hot and fetid and laden with flying, invisible grains of sand.

Tug plunged nervously as the sand whipped his face and flanks. Will kept a firm hand on the reins. Usually, Tug only needed him to hold them lightly but in these conditions, he knew, his horse would respond better to the sense of control that a firm pressure on the bit would impart to him.

'Take it easy, boy,' he said. 'It's just sand.'

The wind was now a living presence around them, keening horribly. And the light was dying. Will was startled to find that Evanlyn, less than five metres away, was now a shadowy, indistinct form in the dimness. The others were no clearer.

Selethen rode in among them and they pressed closer to him to hear him, horses tossing and whinnying nervously. He unwound the protective kheffiyeh from his mouth and shouted his instructions.

'Ride down into the wadi. Dismount and turn your horses' tails to the wind. Try to cover their heads with your cloaks if you can. Then we'll… '

Whatever he was going to add was lost in a giant fit of coughing as he drew in a mouthful of fine flying sand. He doubled over, pulling his headdress across his face again and waving them towards the wadi.

Halt led the way. Will's sense of panic rose as he realised that his mentor would be out of sight in a few metres if he didn't hurry to follow him. He was conscious of other blurred figures close to him as Gilan, Horace, Evanlyn and Svengal all followed suit. Further away there were vague forms moving in the storm and he realised these were the Arridi troops moving to the shelter.

The dim shadow that was Halt and Abelard seemed to sink into the ground and he realised that they must have reached the rim of the wadi. Tug, seeing them disappear, became nervous, sensing that the ground before him was unsafe. He whinnied shrilly and baulked, resisting Will's efforts to urge him forward. The wind was screaming around them, terrifying in its intensity and power, disorienting the little horse. Never before had Tug refused Will's command but now he stood his ground. The wind prevented his hearing the reassuring tones of his trusted master's voice and he sensed danger somewhere ahead. He had seen Halt and Abelard disappear and he was trained to protect his master in situations like this. He braced his legs and stood fast, head down into the screaming, flaying wind.

Will saw the shadowy figure of Horace move past him, recognisable only because of the fact that Kicker stood hands taller than Tug. Someone else moved past him too. He had no idea who it was. Conditions were getting worse, as unbelievable as that might seem. The wind was like the blast from an oven, the air superheated, and the millions of flying, stinging sand particles tore at any piece of exposed skin. The grains forced their way into clothing, under the face masks of the kheffiyehs, into boots, inside collars and into any crevice in the skin – eyelids, ears, nostrils were full of it and Will coughed rackingly.

He found the action of coughing caused him to inhale more sand than he expelled but it was unavoidable.

He couldn't stay here like this, he realised. And he couldn't leave Tug. He would have to dismount and lead the little horse, hoping that the sight of his master in front of him would calm his fears enough for him to move. He took a firm grip on the reins and swung down to the ground. Ordinarily, he would have trusted Tug to stand still when he dismounted. But he knew the little horse was close to panic in this screaming, hellish, sand-laden wind.

He slipped his right arm up under Tug's neck, caressing him and speaking to him, all the time keeping a firm hold on the reins with the other hand. It seemed to be working. Tug's braced forefeet relaxed and he allowed himself to take a faltering few steps in response to Will's urging.

'Come on, boy. It's all right. It's only sand.' He tried to croon the words reassuringly but he was startled by the sound of his own voice, which came out as a dry, faltering croak. He doubted that the horse could hear him but he felt that the contact of his right arm and the proximity of his body was keeping the little horse under control.

He stooped as he led Tug forward, trying to see the point where the ground dropped away into the wadi. It was all he could do to make out the ground itself amid the flying debris of the storm. He glanced up at Tug's face once. The little horse's eyes were tight shut against the wind. Fine sand and dust had crusted over the moisture around the eye sockets and lids.

Where the hell was that bank? He stumbled forward, awkward with the resistant weight of Tug's reluctant body. He pulled the reins firmly and the horse yielded a little, taking three more hesitant steps forward. He realised that Tug's instinct was to turn tail on to the wind, protecting his eyes and nostrils from the whipping sand. But he had to keep forcing the little horse forward to the meagre shelter offered by the wadi's banks. He had a sense that, the storm had not yet reached its peak.

Sand whipped across his eyes, blinding him, and he released his hold around Tug's neck for a moment to try to wipe them clear. It was a futile effort. He gasped and spluttered, blinded and suffocated by the storm. He pulled on 'the reins once more and stepped forward, head bowed against the screaming darkness around him.