Valens lifted his gaze again to search for potential threats. For a moment he could not believe what his eyes saw. He must be mistaken. The hills were moving – like a great brown tidal wave, they surged across the plain towards them.

‘Haboo!’ In his alarm, the Hatrene was shouting in his native tongue.

Iudex translated. ‘Dust storm.’

‘Turn around!’ Hairan had reverted to Latin. ‘Get back to the last wadi. We can shelter there.’

Valens circled his arm. The order was unnecessary. Already the men were yanking the heads of their mounts around. Decimus and Narses were either side of the bell-mare, turning her, the first mules plodding after. Zabda had set off for the rear with the spare horses on leading reins.

Valens could not take his eyes off the approaching storm. A dark wall of swirling cloud, it raced ever closer. Was it a mile away, two? There was no way to tell.

Hairan rode straight past.

‘We best go,’ Iudex said.

Quintus’s horse was playing up, circling as he tried to mount, hopping after it.

‘Let me help,’ Valens said.

‘I am fine. You get moving.’ Quintus’s hair was blowing in the wind, flicking across his face. ‘Stand still, you fucker!’

Iudex was cantering back towards the baggage train.

A spray of sand was hitting Valens in the face, stinging and gritting his eyes. Pebbles were skittering along the ground.

‘Get off, and help with the mules,’ Quintus shouted.

Valens turned his gelding. The mule train was stationary, in disarray. Some of the animals had stepped over the traces and become tangled. Decimus was on foot, wrestling with the twisted leather, shoving the recalcitrant beasts. Narses, still on horseback, was leaning over, gripping the bridle of the bell-mare.

Slinging a leg over the horn of his saddle, Valens dropped to the ground.

‘Have to cut them loose,’ Decimus yelled.

Holding the reins of his horse in one hand, it was near impossible to saw through the thongs.

‘Fuck!’ Decimus was swearing loudly and monotonously. ‘Fuck!’

When the animals were freed, they stood, looking anxious and uncertain.

From the east came a roaring like surf on a shore.

‘Go on, Narses,’ the horse master bellowed. ‘They will follow the bell-mare.’

The Persian needed no further urging, but jammed the heels of his boots into the ribs of his mount. Packs swaying, the mules went after the tinkling bell.

Decimus swung back into the saddle and Valens went to follow his example. He glanced up. The storm loomed over him, no longer a wall, but a high cliff – a hundred foot, or even higher. It roiled with destructive energy, thundered like an avalanche. Stones smacked into his shins. And then it hit.