Publius was taken in. 'Charge! Charge!' he screamed in exultation and his men pushed their mounts harder.

Three of the mercenary cohorts fell even further behind, but Bassius' did not. His soldiers kept pace with the old centurion, now running as if Cerberus himself was after him.

In apparent disarray, the entire Parthian right flank fell back, drawing on the Roman attack. Convinced he had scared them into retreating, Publius heedlessly led the Gauls onward.

He did not see the Parthian commander's gesture.

Almost as one, hundreds of archers turned, drawing their lethal bows to full stretch. With a guttural cry, the officer swept down his arm. Arrows shot forward in a dark swarm, hissing through the air to land with soft thumping sounds. Dozens of Gauls were knocked to the ground. Without pausing for breath, the Parthians loosed for a second time. Feathering man and mount without distinction, the torrent of missiles brought the charge to a juddering halt.

Bassius' men reached the mounds of bodies within moments. It was a horrific sight: the sand covered with dead and injured riders, horses rearing in agony with wooden shafts protruding from chests, rumps, eyes. Many stampeded into the distance, trampling everything underfoot. The deadly rain was still falling, slaughtering the Gauls. Survivors milled about, horseless and bewildered.

Desperately trying to rally his cavalry, Publius was wheeling in circles at the front. Quite abruptly he released the reins and toppled slowly from the saddle, clutching his throat. An arrow had taken him through the neck.

A huge cry of dismay went up from the remaining Gauls.

The situation was hopeless. Brennus realised it at once and looked to the rear, seeking a way out. But it was too late. Hundreds of Parthians were already sweeping round to envelop Bassius' mercenaries and the remnants of Publius' horsemen.

The old centurion had also seen their escape route disappear. 'Form testudo!' he cried.

Discipline still holding, the mercenaries clumped together. Shields clattered off each other, the metal bosses glinting as an armoured square took shape. Men along the sides formed a wall of scuta while those in the middle crouched low, covering their heads completely. The testudo was not an attacking formation, but an extremely effective defensive one – against everything except Parthian arrows.

They watched from behind their shields while the Gauls were cut to pieces. Unable to retreat and unwilling to advance, Publius' cavalry was annihilated before their eyes.

As the last tribesmen fell, warriors began to close in on the testudo. Romulus saw a Parthian jump down beside the body of Crassus' son, knife in hand. There was a huge cheer a few moments later as he stood, Publius' bloody head dangling from his fist. A second warrior rode over and fixed the gory trophy to the tip of his spear.

Fear mushroomed, infecting all. Gazing fixedly at Publius' head, a handful of soldiers broke away from the testudo's protection. They were instantly cut down, striking terror into the rest.

The square wobbled and began to fall apart.

'Close up!' screamed Bassius, but his orders were to no avail. More mercenaries broke free, dropping their heavy shields.

'Publius is dead!' they shouted.

The cohorts behind were still advancing, had not even reached the Parthians. Suddenly the air was filled with cries of panic. Dozens of soldiers appeared through the dust, fleeing in blind panic towards them.

The Cappadocians did what most would do. They turned and ran.

The advance became a retreat as four cohorts bolted heedlessly towards the Roman lines. Straight into another screen of waiting Parthians.

All had fled save the twenty men around Bassius.

'Form testudo!' There was a note of pride in the senior centurion's voice.

Romulus, Brennus, Tarquinius and the remaining mercenaries moved closer to make a small square.

'Roman soldiers do not run!' Bassius yelled. 'Especially when the whole army is watching!' He pointed at the enemy. 'We will stand and fight!'

Through clouds of sand and grit, Romulus saw Parthians riding rings round the fleeing mercenaries. Arrows scythed through the air, cutting them down. Curved swords flashed in the sunlight, opening gaping wounds in men's backs. Hooves trampled the fallen into the sand, face down. Few of the terrified soldiers even lifted their weapons to retaliate.

The group watched helplessly as what had been a rout now became a slaughter. It was over very quickly. Except for those huddled with Bassius, Publius' cavalry and the four cohorts had been completely destroyed in a stunning example of battle tactics.

The sun beat down, unrelenting. Not a cloud was visible. The air was windless. Oppressive. Dead.

Under the raised scuta, the temperature was climbing fast. It would soon be unbearable. But Parthian arrows awaited any who stood up.

'Anyone got water?' asked Felix hopefully. The little Gaul who shared the friends' tent was one of the few to stand fast.

Romulus handed over his water bag, still a quarter full.

Felix took a mouthful and passed it back. 'That won't last much longer.'

'Doesn't need to,' muttered one of the others. 'Elysium is waiting for us.'

'We'll take plenty of them too,' said Felix grimly.

'That's the spirit,' bellowed Bassius.

Hearing this, the mercenaries roared at the tops of their voices. They would die bravely. Like warriors. Like Romans.

Horrifying screams echoed all around them as wounded men thrashed about. Blood saturated the yellow sand, turning it a deep red. Innumerable corpses lay scattered like broken dolls.

Crouching behind shields they now knew to be useless, the survivors waited for the inevitable attack. As the dust began to settle, hundreds of Parthians rode in from all sides. They were boxed in completely.

But no arrows were launched as a lone rider in fine robes rode towards the testudo, his horse picking its way delicately between the bodies. The Parthian officer reined in at a safe distance and watched them, his eyes inscrutable.

'Bastards!' cried Bassius. 'Come and get us!'

As Romulus and his comrades screamed their rage and defiance, he and Brennus exchanged a meaningful look. When the Parthian gave the order, death would take all of them. It would be no glorious end – just a volley from the lethal composite bows. There would still be no surrender.

Farewell, Mother. The gods be with you, Fabiola.

A journey beyond where any Allobroge has gone. And here at least I can die without having to run from my loved ones.

The dark-skinned man stared long and hard. Outnumbered and surrounded by mounds of their own dead, his enemies still had not laid down their weapons. Speaking in an unfamiliar tongue, he pointed back towards Crassus' army.

'What is he saying?'

'Probably telling us to run. Son of a whore,' said Felix, curling his lip. 'So they can kill us too.'

The Parthian gestured again at the Roman lines.

Tarquinius turned to Bassius. 'We can go, sir.'

The senior centurion regarded him blankly while the others gaped.

'You understand him?' hissed Romulus.

'Parthian is very similar to ancient Etruscan,' he muttered.

'The bastards could have killed us five times over,' admitted Bassius.

Tarquinius called out in the same language and the officer listened carefully before replying.

With raised eyebrows, Bassius waited until the brief conversation had finished. 'What was that about, Optio?'

'I asked him who he was, sir.'

'And?'

'He is Surena, the leader of the Parthian army.'

There was a collective sharp intake of breath.

Tarquinius raised his voice. 'Surena said we are all brave men, who do not deserve to die today. He is giving us safe passage.'