At last Crassus came to his senses and took in the approaching enemy. 'Close order!' he croaked. 'Launch javelins at twenty paces. No more!'

The messenger by his side scuttled over to the trumpeters. If the orders weren't relayed fast, the Parthians would be on them.

'What then, general?' One tribune had plucked up enough courage to speak.

Surprised rather than angry, Crassus waved his hands vaguely in the air. 'Weather this attack. Shower the Parthians with pila. That'll drive them off.'

The tribune looked confused. 'But their arrows have a greater range than javelins.'

'Do as I say,' said Crassus dully. 'Nothing can withstand Rome's legions.'

The officer withdrew, eyes bulging with alarm.

Crassus had lost his mind.

Unsure exactly where to go, Bassius led his men to the position held by the Sixth Legion, right of the Roman centre.

'You've no time to reach the other mercenaries,' shouted a centurion as they came closer. 'Against regulations, but bring your boys in alongside mine.'

'Very good, comrade. You heard the officer!' Bassius ordered. 'Six men wide, three deep. Move!'

The group quickly formed up beside the regulars. The barrel-chested centurion who had spoken leaned over to grip Bassius' forearm.

'Gaius Peregrinus Sido. First Centurion, First Cohort.'

'Marcus Aemilius Bassius. Senior Centurion, Fourth Cohort of Gaulish mercenaries. And veteran of the Fifth.'

'That was a massacre out there,' said Sido. 'You did well to survive.'

'The bastards led us into a trap, pure and simple. Their right flank fled, then they swept round and enveloped us. Publius never saw it coming.'

Sido whistled with respect. 'Why are you not dead?'

'We didn't run like the rest,' shrugged Bassius. 'And the Parthian leader let us go.'

'Mars above! That should get you a few drinks back home.'

'I hope so,' laughed Bassius grimly, eyeing the Parthian archers. It would only be moments before they reached the Roman lines.

'Our pila don't have the range of their bows,' said Sido heavily. 'What can we do?'

'We'll need to hold the bastards off till sunset,' replied Bassius. 'Then fall back to Carrhae under cover of darkness and head for the mountains tomorrow.'

'Retreat?' Sido sighed. 'We can't fight those sons of whores in the open, that's for sure.'

'Crassus had better see it that way damn quick, or it will mean death for all of us.'

Since the cataphracts had ridden past, there had been no commands from the centre. Finally the bucinae blared a series of short notes.

'Close ranks! Prepare for attack!'

The men at the front needed no prompting. Shields slammed together while the soldiers behind held theirs angled overhead. There was nothing else to do. Legionary scuta could resist normal missiles, but as every man knew only too well, the Parthian bows were different.

Clouds of dust rose from the horses, filling the atmosphere with a fine choking powder. With the Romans in a continuous line, the archers were unable to ride around each cohort as before. Now they would have to ride along the enemy's front and far fewer could attack at any one time.

This provided Crassus' legions with only a shade more respite. A wave of riders swept in, releasing hundreds of shafts from fifty paces. The Roman officers did not order volleys of javelins. There was no point. As the Parthian assault withdrew, it was immediately replaced by another. Storms of arrows rained upon on the beleaguered army, piercing wood, metal and flesh without distinction.

Screams of pain rose up as the barbed tips penetrated scuta, taking out eyes and pinning feet to the sand. And every soldier that fell created a hole in the shield wall. Into these gaps came scores more missiles, the Parthians using every opportunity to decimate their foes. The Romans cowered under their shields with gritted teeth, praying.

Several of Bassius' mercenaries fell wounded in the prolonged onslaught. Following the centurion's lead, the others snapped the shafts off and pulled them out when they could. Men roared in agony as blood poured from their wounds. The air was filled with the moans, galloping hooves and the hiss of feathered shafts: a terrifying cacophony.

Romulus had grown used to the shrieking, but the number of combatants was far greater than he could have ever imagined. This was death on a grand scale, the sheer magnitude of slaughter impossible to comprehend. Cannae must have been something like this, he thought. A battle that the Republic had lost.

The attacks lasted as long as the enemy had arrows. Whenever the Parthians had exhausted their supply, they simply rode back to the camel train for more. There were enough archers to ensure that any breaks were few and far between. At various stages, the frustrated centurions ordered javelins be thrown, but the horsemen were rarely close enough. Hundreds of pila flew through the air to land on the sand, wasted and useless.

After hours of this endless cycle, Roman morale was falling fast. In the ranks of the Sixth alone, nearly a thousand men had been killed. Hundreds more lay injured on the baking hot sand. The air was now thick with dread and the officers were finding it increasingly difficult to keep their units in position.

On the left wing, the Iberian cavalry had fled, unwilling to suffer the same fate as the Gauls. With no sign of Ariamnes and his Nabataeans, the Romans retained no horsemen at all. The rest of Crassus' army had been battered to a pulp, left unable to respond in any way.

Cohort after cohort stood reeling under the onslaught. Parched. Exhausted. Wavering. And about to run.

But instead of another attack, the drums and bells began to sound. While the noise rose in an unearthly crescendo, the horse archers pulled back. Unsure what was happening, the uninjured Roman soldiers waited, their nerves wire-taut. Thanks to the dust cloud that had taken up a permanent place between the two forces, the Parthian army was invisible to them.

For what seemed an eternity, nothing happened.

Then the instruments fell abruptly silent. Surena was a shrewd judge of men and it was time for the hammer blow.

Beneath Romulus' feet the sand began to tremble. Still nothing could be discerned before them.

Then he knew.

'Cataphracts!'

The senior centurion stared at Romulus blankly.

'A charge by heavy cavalry, sir!'

Bassius turned to Sido and swore. 'They will smash us apart! Everyone still with pila to the front.'

The other centurion nodded jerkily. He had seen the cataphracts and could well imagine their capability.

'All men with javelins move forward! Hurry!'

Brennus pushed his way through, keen to get to grips with the enemy. He was sure now that his journey was being watched over by the gods themselves. Therefore there was a purpose to it – to all he had sacrificed. Now it was time to fight.

Having thrown their pila already, Romulus and Tarquinius stayed put.

'Other ranks, close up,' ordered Bassius. 'Use your spears to stab the horses' bellies. Gut them! Take their fucking eyes out! Rip the riders off!'

'Stand fast!' Sido raised a bloody gladius in the air. 'For Rome!'

The soldiers managed a ragged cheer and hurriedly formed up. Romulus and Tarquinius found themselves in the second rank, a few paces behind Brennus. The Gaul had elbowed his way to stand near the two centurions.

The ground shook from the drumming of hooves and a low thunder filled the air. Bassius had just enough time to shout, 'Shields up! Pila ready!' before the Parthians emerged from the concealing gloom. Riding in a wedge formation, the desert horsemen were already at full gallop. In response to a shouted order, their heavy lances lowered as one. The centurions had no chance to order a volley of javelins. With devastating force, a thousand heavy cavalry punched into the Roman lines. Sido and those at the front were smashed aside or trampled underfoot while the men behind received a lance in the chest.