The summons by the Leader of the Strangers had come like a peal of thunder from a clear sky, like a portent. Although the headquarters of the frumentarii lay at no distance from the camp of his own unit on the Caelian Hill, Valens had never set foot in the place. No one in their right mind chose to go there. Like all members of the Roman upper classes, Valens both despised and feared the frumentarii. The secret soldiers were underhand spies and assassins. The Emperors used them to pry into the lives of the elite, to ferret out any hint of disloyalty. Dressed in civilian clothing, they were rumoured to be impossible to recognise. Nowhere was safe from them. An ill-considered word in a casual conversation at the baths, or at a dinner party, might lead to arrest. If evidence was lacking, the frumentarii were notorious for inventing incriminating details. Once in the hands of the torturers, you would confess to anything.

Murena, the Princeps Peregrinorum, was a vulgar little man, obviously promoted from the ranks. But he had been civil. Once he had established that Valens was indeed Marcus Aelius Valens, Deputy Tribune in the Imperial Horse Guards, he had been brisk and efficient. It was well known, Murena had said, that Valens was bored by garrison duty in the capital. Valens had agreed. He had not wondered how Murena knew. It was the sort of thing the frumentarii found out.

Another officer was introduced, a tall man in middle age, with a short beard and protruding ears. Gnaeus Claudius Severus was to lead a team of ten across Mesopotamia, and into Persian territory. Severus would claim to be a merchant, trading in jewellery. Eight frumentarii would act as the guards of his caravan. Valens would be second in command, and pose as Severus’s nephew. The aim of the mission was to rescue a Persian prince named Sasan from a prison called the Castle of Silence, situated to the south-east of the Caspian Sea. Once they had freed the Prince, they were to travel north to the Steppe. There, the ruler of the nomad Heruli would convey them to the Black Sea, from where they would take passage on a ship back to the empire.

‘Any questions?’ Murena had asked.

Dozens jostled in Valens’s mind: routes, logistics, money, weapons, above all the chances of survival.

‘No, sir,’ he had replied.

‘Good,’ Murena had said. ‘You will report to Severus in the military camp at Zeugma on the Euphrates on the Kalends of August. You will travel there alone, not even with a slave.’

‘Sir.’