Above all, would they blunder into an enemy patrol or outpost? The Sassanids were alerted. The cavalry out on the plain the day before left no doubt of that. For a long time Valens had half suspected there was a traitor among the ten that had ridden out of Zeugma. Too much had gone wrong. The bandits killing Severus, the death of Quintus the navigator in the dust storm – so suspicious in retrospect – the trail left for the tent-dwellers to follow. Now the Sassanid horsemen waiting for them to come down from the mountains had near convinced him. How in Hades had the news preceded them down from the Castle of Silence? And who was the malignant presence in the surviving members of the brotherhood? It was not Narses. Of that Valens was quite certain. The Persian could have betrayed them at any point. All he would have needed to do was inform Naduk when they rode into the fortress-prison. Surely it was not Hairan? The young Hatrene had every reason to loathe everything Persian. They had killed his family, laid waste his home. There was Iudex. He was mad. There was no telling what his god might put into his mind. What possible reason might have inspired the other three? That ugly old pederast Aulus had an insubordinate nature. The no younger or better-favoured Clemens was likewise unruly, and was far more aggressive. Decimus, better preserved and more handsome, gave no reason to think that he was unsatisfied with the world. Valens would have thought Zabda the likeliest candidate. But Zabda, of course, was dead. And he had died saving Valens’s life.

In the ambient light, Valens saw Narses raise his hand to call a halt. The Persian turned his horse, and handed the reins to Valens.

‘Pass the word to wait,’ Narses hissed, his mouth so close to Valens that the Persian’s beard tickled his face. ‘The levee is just ahead. I will go and look.’

As he went, Narses made no more noise than a fox would going along the narrow track slightly raised above the water.

Soon there was nothing to hear but the wind soughing through the reeds.

Narses’s big Nisean stallion lifted its feet unhappily from the mud. Valens went to quiet it, but the black horse tried to bite his arm. Valens growled quietly. The stallion eyed him warily, but stopped playing up.

A gasping, choking sound, then a paroxysm of coughing rent the air.

At once the peace of the night was shattered. Persian voices shouted. A trumpet rang out. A light flared ahead. Then more appeared, one after another, running in a line like fire along a row of stubble.

The levee was less than a hundred paces to the north. By the light of the torches, Valens could see the Sassanids on top of it. There were dozens of them spaced in a cordon along the walkway. They were on foot, their horses picketed.