‘You all know our goal,’ Severus said, ‘but not our route.’

He unrolled a map. It showed the usual cobweb of roads linking pictograms of settlements, with scant indication of topography, and, away from the roads, only an inexact relationship with geography.

‘We will take the road across Mesopotamia through Batnae and Carrhae. Beyond Resaina, we will leave the path and cross the desert directly into Persian territory.’ Severus pointed to an ochre-coloured section of the map, completely empty except for the word ‘Arbayestan’. ‘Quintus will plot our course. He served with the fleet in the Mediterranean at Ravenna, and on the Rhine along the German border. He can navigate by the stars.’

For some reason that Valens could not guess, Iudex looked far from happy.

‘When we reach Adiabene and the mountains of Matiane, Narses will act as our guide to the shores of the Caspian Sea, and on to the Castle of Silence.’

‘Is that wise?’ said Clemens.

Severus looked at the armourer.

‘Orientals are not to be trusted. I should know, I served in the east for years. As for this Persian, is it wise to entrust ourselves to a renegade to lead us into the realm of the King of Kings? Blood is thicker than water. He will return to his own kind, and deliver us, hands bound, to their tortures.’

Narses leant forward, his dark eyes glittering above his beard. ‘It is true I served the King of Kings Shapur faithfully. After the battle of Corycus, my lord Zik Zabrigan released me from my oath. At that place I took the sacrametum to Gallienus. For five years I have fought for Rome.’

Clemens snorted. ‘You were trapped by the Roman forces under Ballista. You had no choice. Anyway, a man who has changed sides once finds it easy to do so again. Oaths are just words to you Persians. Deceit is in your nature.’

The Persian was half out of his seat. ‘I was raised in the clan Suren; taught to ride, shoot the bow, and tell the truth. The god Mazda instructs us to abhor the Lie.’

Like a stern Roman of old, the lined and careworn face of Clemens expressed nothing but contempt for such foreign concepts. ‘You fuck your own mothers, let the beasts eat the corpses of your fathers. The gods themselves avert their eyes from your crimes.’

‘Enough!’ Severus snapped. ‘Every people follow their own customs. Our chances of success are slim enough. If we are divided among ourselves, we are doomed to failure. No matter the origins of a soldier, nothing is more sacred than the military oath. Here and now, before each other and the gods, you will reaffirm your vows.’