A Sassanid war drum thundered. The clibanarii drew their composite bows. A flight of three thousand arrows shot blind into the gloom. Like rain blown in from the sea, a dark squall of shafts came back. Here and there among the clibanarii a horse plunged as an arrow tip found its way through mail, plate and hardened leather. The Persian light horse joined the exchange, aiming a high trajectory over the heads of the armoured men. The arrow storm intensified. Above the thrum of numberless arrows came the screams of men and horses. Men were dying in the Persian ranks. Out of sight, men would be falling among the Alani. There was something uncanny about this fight with an unseen enemy.

‘This cannot last,’ Rutilus said. ‘Their quivers will soon be empty.’

‘It is impossible to tell, but the Persians should be getting the better of it,’ Castricius said. ‘Their armour will be heavier than the nomads. The Alani will have to do something.’

As if in response to their words, the incoming arrows slackened. Dark shapes emerged at the front of the wall of fog. A frenzy of horns, drums and yells sounded from the enemy.

‘Here they come,’ Maximus said.

Three closepacked wedges of horsemen burst from the curtain of moisture. Strange standards flew above: animal skulls, pelts, horse tails, the outstretched wings of birds of prey. Tatters of mist swirled about them.

The standard of Narseh inclined forward, trumpets blared, the war drums beat faster. The mighty Nisean chargers stepped out. Like a great wave building, ponderous but terrible, the Persian force surged towards the foe.

The nomads covered the ground fast. The Sassanids were still at a slow walk when the forces collided. The noise of the clash rolled back down the valley to the Romans watching on the roof. The Alani were outnumbered, but momentum drove the tips of their wedges into the Persian formation. The hideous cacophony of combat stunned the senses.

The lilac standard of Narseh dipped – seemed like to fall – then straightened. The fighting was fiercest around the Sassanid prince. The Alani advance here slowed as Persian numbers told. The other two wedges were already stationary. A great roar went up. Narseh and his retainers had stifled the central thrust of the nomads.

Across the valley, the combatants were pressed close. Often with no room to wield spears or swords, men wrestled on horseback. Clawing with their fingers at each other’s throats, gouging their eyes, seeking to fling them down among the stamping hooves.

‘More like an infantry battle,’ Castricius said.

‘Unless they cut Narseh down, the nomads will lose,’ Rutilus said.

The nomads fought with ferocity, but it could not last. The collapse started at the rear, among those still uncommitted. In ones and twos, then in small clumps, finally in whole groups, nomads pulled their horses’ heads around and bolted back up the valley into the obscurity of the fog.

Peroz! Peroz! Screaming victory, the Sassanids and their allies – heavy cavalry and light horse – poured after them.

As if swept by the hand of a deity, the battlefield was empty. There was the inevitable detritus of war – broken and discarded weapons, dead and injured men and beasts, unscrupulous and avaricious men from the victors, men of no honour, already dismounted and scavenging the field – but the combatants were gone.

The watchers on the tower were silent. There seemed nothing to say. The fog had receded further. It still hung on the hilltops, made a ceiling to the valley. Yet now Ballista could see almost a mile or so up the valley. It was not far enough to see the rout. Everything was eerily quiet. They could hear the river. It ran on as before. From there, or somewhere, came the sound of frogs: brekeke-kex.

The first vultures were dropping down on to the stricken field. Some Suani were slinking out from Dikaiosyne to join those robbing the dead and sending the wounded to join them, so they could take what they had also. Persians were said to carry all their wealth on them. Their allies the Suani would go to them first.

‘Is it all over?’ Pythonissa asked.

‘Yes,’ said Castricius. ‘It is hard to believe thousands of men are being slaughtered just up there.’

A movement caught Ballista’s eye.

‘Fuck,’ Maximus said.

Half a mile away, in the gully to the right, where a tributary came down to the Alontas, the trees and bushes were moving. There was no wind.

‘Fuck,’ Maximus said.

The dark, hunched shapes of the steppe ponies and their riders moved out on to the floor of the valley. They milled for a time. Five hundred, a thousand – the exact number was hard to tell. With a whoop, the majority rode away to the north and vanished into the mist. They were behind the advancing Persians. Their arrival would be a complete surprise, most likely change the entire course of events. It was a perfect ambush. The Alani charge and withdrawal had been planned from the start. All the time the ambushers in the gulley had been waiting their moment.

About two hundred Alani remained in sight. In no particular order, they trotted south towards Dikaiosyne. They halted in a rough line about a hundred yards from the village.

Ballista turned to Pythonissa. ‘How many armed Suani do you have here?’

‘Around three hundred.’ She was admirably calm.

‘How many of them have horses?’

‘One in ten.’

‘Have the mounted gather in the village square. Those on foot must block the entrances to the alleyways that face north.’

She told an attendant to see to it. ‘What are you going to do?’ she asked Ballista.

‘Talk to Hamazasp, then the mobad Manzik.’

‘Hamazasp will kill you.’

‘I will have the others with me.’ He pointed to Maximus, Castricius and Rutilus.

‘It is not enough. Ten of my mounted Suani are below; take them with you.’

The king of Iberia was quartered in another tower facing north. It was surrounded by his warriors. They did not seem unduly concerned by what had happened. They eyed the men who rode up with some hostility.

Ballista addressed a man in elaborate armour, obviously a leader. ‘I need to speak to Hamazasp.’

‘He is ill.’ The man spoke Greek with a heavy accent; his tone was dismissive.

‘I need to speak to him.’

‘No.’

‘If you do not tell him, Narseh will have you impaled.’

‘Narseh might not return. You are in no position to issue threats, Roman.’ He touched the hilt of his sword. His men shifted.

A figure appeared on the roof of the tower, looking down. It was the Iberian king. He did not speak.

‘Hamazasp,’ Ballista called up, ‘you must lead the warriors with you. We can brush aside the nomads before the village. If we are quick, we can save the day.’

Hamazasp stared down at Ballista with loathing. Still he did not speak. Then he turned away and was gone.

‘Not long for you now, Kinaidos.’ The Iberian laughed.

Ballista swallowed a retort to the insult – that bastard Hamazasp would suffer for saying he took it up the arse. Ballista backed his horse. The others did the same. When they were out of weapon reach, they wheeled and rode away.

Appropriately enough, Manzik the mobad was praying when they arrived at the house he had taken. He finished as Ballista burst into the courtyard and said what he wanted.

‘I am afraid I cannot lead the Sassanids,’ Manzik said. ‘We mobads , with our own hands, can kill everything – ants, snakes, anything that walks, crawls or flies – we take pride in it. But we are forbidden to kill dogs or men.’

‘Tell your men to take orders from me.’ Ballista knew time was fast running out.

‘What about the baggage? Prince Narseh ordered us to guard it.’

‘If the army is defeated, the baggage will be the least of our concerns.’

‘Of course, you are right. Take the men. I will remain and attempt to protect the property of the prince and the warriors.’

Ballista had just over a hundred and thirty mounted men: Persians, some Suani, just four Romans. All well mounted, with good armour: enough for the first task. The second was another matter. He divided the cavalry into two columns, each waiting out of sight in an alley. Ballista was at the head of one, Rutilus the other. Castricius was to bring up the Suani foot. Pythonissa had been told to barricade herself in her house. Allfather knew if she would.

The Alani out in the valley before the village were not expecting trouble. Their line had disintegrated. Apart from forty or so gathered around a ragged standard in the centre, most had dismounted and were looting the dead. Even those still on horse had dropped their reins, were sitting all unconcerned; drinking, eating, chatting.

‘Now!’ Ballista said, and kicked his heels into the flanks of his horse. Behind him, a Sassanid trumpet relayed the order. The Suani infantry, who had been blocking the mouth of the lane, leapt aside at the last moment. Ballista’s mount accelerated out into the open. There was a reassuring thunder of hooves behind. To his left, he saw the tall figure of Rutilus leading the other charge.

The nomads dropped their pickings, ran for their ponies, swung up into the saddle. All too late. Ballista saw the closest of the looters go down under Rutilus’s blade. The Alani around the standard were not caught so unprepared. A few managed to loose off arrows. They shrieked through the air, but none came close to Ballista. The nomads dragged swords clear of scabbards, made to stand up to the charge.

Ballista splashed through a strand of the Alontas and put the big Nisean charger straight at the pony of the Alan chief. The collision sent the shaggy small beast, still snapping, back on its haunches. The chief fought to retain his seat. Surging past, Ballista swung his blade overhand. The chief instinctively flung up an arm to protect his head. Ballista’s blade severed it below the elbow.