The coughing did not stop.

A Persian officer, mounted and gorgeous in silks, shouted, obviously for silence.

The noise from the levee dropped.

Silence from the marsh. Then another burst of racking coughing.

It seemed to Valens that the Sassanid officer pointed directly at him through the darkness. From back down the column came a commotion. A rider was scrambling into the saddle.

‘Get down!’ someone snapped.

Barbad was mounted. With a savage kick, the old eunuch forced his horse off the track. It landed belly down in the stagnant pool with a terrible splash. Half swimming, half running, it breasted the surface. The glossy black water foamed white in its wake.

‘No, Barbad!’

Just behind Valens, the boy was yelling as he heaved himself up into the saddle.

Dropping the reins, Valens lunged at the boy. Catching him by the scruff of the neck, he hauled him back to the ground.

The eunuch’s horse was floundering east towards an islet with a stand of trees.

Prince Sasan filled his lungs to shout. Valens clapped a hand over his mouth.

The three loose horses were sidling, thinking about bolting.

‘Someone catch them,’ Valens said.

Hairan moved past.

Sasan bit Valen’s hand. The sharp little teeth sank into his palm. Instinctively, Valens let go. The boy darted after the eunuch.

Valens went into the fen a moment after Sasan. The water was up to the boy’s chest. Undeterred, he was forcing his way through. Longer legged, Valens closed the distance in a couple of strides. He dived to tackle the child. They both went under.

The cold black sludge enveloped Valens. It forced its way into his mouth and ears. He could see nothing. For a hideous moment, neither his hands or his feet touched the bottom. Then a boot hit something solid, and he came up into the air.

The boy was nowhere in sight.

Looking round wildly, Valens saw a disturbance on the surface. He reached down, groping. His hand closed on something. He hauled Sasan out of the swamp.

Gasping, the boy was going to shout again.

Valens cuffed him hard around the ear. ‘Be silent, you fool. The old man is sacrificing his life for yours.’

A flash of movement in the moonlight, and Barbad raced into the deep shade of the trees. A shout of command from the levee was followed by the sound of men wading down into the swamp.

‘Do not move,’ Valens whispered.

He and the boy stood in the stinking mire.

The others were frozen under the reeds along the track.

More splashing and crashing indicated Barbad’s mad flight had crossed the tiny island and re-entered the marsh.

A jabber of Persian voices. The bobbing light of torches trailing after the eunuch.

It was cold in the water. Valens felt Sasan shivering. He put his arms around the boy.

The torches swarmed up onto the islet and disappeared under the trees.