‘What can go wrong?’ Narses said. ‘We are serving with a man who visits the underworld and talks to the gods.’

Iudex ignored the Persian’s mockery. ‘There is divine light in all humanity. When unbelievers die, it is not lost forever as Mani holds. As their corpses decompose, the light is released into the earth, and from there enters the plants. When these are eaten by the elect, it is released into the heavens.’

‘Well that accounts for your continual farting.’ Narses was laughing openly. ‘It is a fine religion that sees flatulence as a sacrament.’

Iudex was undisturbed. ‘Those who are deceived by other religions find no escape. They go blindly to destruction. The sooner the unredeemed die the better.’

‘And what if you kill someone who is redeemed?’ Narses said.

‘Then he will go straight to heaven.’

‘Enough,’ Valens said. ‘He is coming.’

A Persian soldier was carrying a tray across the courtyard from the kitchens.

Narses got to his feet, and hailed the man in Persian. They spoke for a time. At first the soldier looked uncertain, but then he laughed.

Narses turned to Valens, and spoke in Greek. ‘Get up, you lazy Roman dog. It is unfitting for a Persian warrior to carry things like a servant, while a prisoner sits warming his arse by the brazier.’

Valens got up.

The Persian said something.

‘You are not going in there armed,’ Narses said.

Reluctantly, Valens unbuckled his sword belt and handed it to Iudex.

‘Look lively,’ Narses snapped. ‘Take the tray and follow the soldier.’

Valens did as he was commanded. There was bread and cheese, some cold meat and fruit, and a metal flagon of wine. It was a lot of food for a child.

Narses spoke again to Valens. Once more the tone was haughty, but this time the words were in Latin. ‘Everything will be ready. We will be waiting.’

Valens trudged up the steps. At the top, the Persian unlocked the door. When they had gone through, he locked it behind them and put the keys back on his belt.

It was dark and cold within the tower. Stone stairs ran around the inside wall. Every so often, a lamp burning in a niche gave a feeble light. On the second floor they came out onto a landing. A narrow window, unshuttered but set with iron bars, faced a closed door. They did not stop there, or on the third floor.

The tread of their boots echoed in the gloom as they went up towards the top floor.

A voice called down from above.

The Persian replied.

The guard was leaning on the frame of the final door.