They left at dawn, and by the afternoon any remaining doubts Valens had entertained about not having left Rome wholly behind were dispelled. A multitude of large black birds wheeled around the summit of a lonely hill to the south. At the apex of the outcrop was the silhouette of a curved building.

‘Ahuramazda be praised,’ Narses said.

With one accord, as if summoned by that deity, they turned their animals away from the stream and up the incline.

The walls shone golden in the sunlight. Twenty or more of the birds sat on the wall like dark sentinels. A ramp ran up to an open gateway at the top of the building. Dismounting at its base, they hobbled the horses. The mules they left standing in a line, the leading reins between them trailing on the ground.

The birds on the wall did not take flight, but watched them without curiosity or alarm as they ascended the walkway. The clean desert air was tainted at the top of the ramp. It was worse in the darkness under the gate. When they got inside, they saw the horror.

They were standing on a sort of viewing platform. The building was round, with a flat roof. A low parapet ran around the edge, and there was a pit in the centre. The corpses were laid out in concentric circles around the pit: the small bodies of children innermost, then women, the men at the outside. There were forty or more, in all stages of decomposition. Some were little more than bones, ready to be shovelled into the pit. Others appeared to have been dead for only a few days.

Valens found himself staring at a nearby corpse. The man was on his back, his legs splayed in a vaguely obscene way. Strips of clothing, just tattered rags, still clung to the putrefying flesh. A vulture was tearing at the meat of his calf. The bird looked up malevolently, as if the intruders might dispute its food.

‘Everywhere, custom is king,’ Valens said.

‘What?’ Clemens said. The question was peremptory, the armourer’s voice tight.

‘A story in Herodotus. A Persian king asked some of his Greek subjects what they would take to expose the bodies of their dead. They refused, and begged him not to even say such a thing. He asked some of his own people if they would bury their dead. The response was the same. The moral is that people everywhere believe their own customs are natural.’

‘I don’t give a fuck about some old Greek.’ Clemens’s jowls were quivering with disgust. ‘No Roman would ever contemplate such a thing. Only savages could treat their own with such inhumanity.’

‘This is a holy place, a tower of silence,’ Narses said.

‘It is revolting,’ Clemens almost shouted. ‘A crime against the gods and men!’

‘It is the will of the Lord of Light, Ahuramazda.’ Narses spoke softly.