That was where Murena had seen Severus. At the start of Valerian’s ill-fated campaign, somewhere north of Carrhae, he had delivered a report to Severus and his Thracians. He remembered him now – a tough, experienced officer.

‘Since then you have served as a procurator, overseeing imperial estates in the provinces of Arabia and Macedonia.’ Volusianus had consulted no notes, but spoke from memory.

‘Yes, sir.’

‘Are you ready to return to the standards?’

‘Ready, sir.’

With an energy that defied his age, Volusianus bounded to his feet. Smiling, he extended his hand. Severus shook it.

‘Have a seat, Severus.’

The tension had gone from the room.

Pouring the officer a cup of wine, Volusianus settled himself back in his chair. He exuded the amiability of a farmer at a festival.

‘Health, and great joy.’

They toasted each other.

‘Am I right in thinking that growing up in the east, Severus’ – somehow the name was pronounced as if to an intimate friend – ‘you speak Greek, as well as a smattering of Syriac and Persian?’ Volusianus was beaming like the sun.

‘I do, sir.’

‘Then you are the man that I am looking for.’ Volusianus dropped his air of bucolic geniality with the speed and finality of a theatre curtain. ‘Tell me what you know of the Castle of Silence.’

Severus narrowed his eyes with the effort of recall. ‘A Persian fortress, somewhere in the mountains south-east of the Caspian Sea. They use it as a prison.’

‘Quite so,’ Volusianus said.

Murena wanted to add that for Persians it was a capital crime to mention the name of anyone condemned there, but he did not. A frumentarius must know when to hold his tongue.

‘Prince Sasan, a nephew of the King of Kings, has been confined in the fortress.’ Volusianus gave Severus a measuring stare. ‘You will go and get him out.’

It was to Severus’s credit that he received this order with the calm of a Stoic philosopher instructed to take his own life. The command amounted to much the same.

‘What troops would be at my disposal, sir?’ he asked after a moment.

‘Ten men. The leader of the frumentarii,’ Volusianus indicated Murena, ‘will provide you with a squad of eight of his best soldiers, and a junior officer of equestrian status.’

Severus glanced at Murena.

‘You will be disguised as a merchant caravan out of Antioch,’ Volusianus continued. ‘Murena here will arrange for mounts, baggage animals and suitable trade goods. Any questions so far?’

Severus frowned with concentration. ‘When we have freed the prince, how will we return to the empire? After the rescue there will be a pursuit. Our subterfuge will be exposed. We will not be able to cross back into Syria posing as merchants.’

‘You will not return to Syria,’ Volusianus said. ‘Murena, get a decent map.’