Suddenly the sound stopped and the ground ceased shaking. The roar of the flames behind us seemed to still for a moment and even the wind grew calm. I have seen this before, and know it to be but the false tranquillity of an enemy gathering itself for the onslaught.

'Stand your ground!' shouted Arthur. 'Here they come!'

We stood gazing into the darkness, the fire at our backs throwing our shadows before us like an all-encircling army of shape-shifting phantoms.

Breathless, we waited.

Across the meadow, the trees began thrashing back and forth as if in the grip of a violent storm, but the air remained still. I heard a low, grinding sound and the trees parted, lying down on either side as if divided by a giant hand.

In the same instant, the burning oak behind us gave another tremendous crack, sending sparks and chunks of flaming wood showering all around. The fire at our backs leapt high, and higher still into the night; our shadows flickered and danced out across the darkened meadow. In the newly opened gap where the forest met the river, a figure appeared – a lone warrior on a horse.

'There!' someone shouted, and from the corner of my eye I saw a movement as the speaker thrust a hand to point out the horseman advancing towards us.

'Do not break the circle!' Myrddin Emrys cried, his voice terrible in the silence. 'As God is life and evil death, hold tight and do not let go!'

The rider came on, slowly. He carried a dark shield with a burnished iron rim; both the shield's rim and the honed tip of an upright spear glimmered in the firelight, and the blade on his thigh gleamed dull red. The warrior was dressed all in black from head to foot, and wore a hooded cloak, so I could not see his face; from the withers and flanks of the horse, long black strips of fine cloth rippled and fluttered as the animal moved, making it seem as if the beast were floating towards us.

The dark rider advanced to within a spear's cast of us, whereupon the Emrys challenged him. 'Halt!' he shouted in his voice of command. The Swift Sure Hand is over us. You can do no evil here. Go back.'

The rider made no reply, but sat regarding us while his mount chafed the ground impatiently.

'Go back to the hell from whence you came,' Myrddin shouted again. 'You cannot harm us.'

By way of reply, the warrior shifted the shield to cover his chest and, with the slightest lifting of the reins, turned the horse and began riding around the ring. He made one circuit, then another and another, slowly gathering pace with each pass. By the sixth or seventh circuit, the horse had reached an easy canter.

Around and around he rode, in a long, slow circle, the hooves of his mount beating the ground in a rhythmic thump like the rising beat of a drum. Around and around – the canter became a trot… the trot became a gallop… the gallop became faster, the beat of the hooves coming quicker.

The strange black strips of cloth hanging from the horse's sides rustled like wings. I could hear the beast's breath coming in snorts and gasps now as the pace began to tell. The warrior's cloak billowed out behind him and the hood slipped from his head, revealing a face I knew well.

'Llenlleawg!'

It was Arthur, crying out in surprise and dismay. He shouted again, hoping, I think, to gain his former champion's attention. Others quickly joined in, and soon everyone was calling Llenlleawg's name. I shouted, too, thinking that we might yet sway him from his course.

But looking neither right nor left, the Irish champion urged his mount to charging speed and lowered the spear.

'Stand your ground, men!' shouted Arthur. 'Do not break the circle!'

Even before he finished speaking, I saw the quick flick of the reins and the horse swerved towards the ring of Cymbrogi, driving in towards the ring at a shallow angle to my right. The spear swung over the horse's neck and came level. The Cymbrogi, arms linked, shouted to distract the horse, and braced themselves for the killing blow.

But the attack was merely a feint, and he slanted away well before committing himself to the charge.

'The Swift Sure Hand upholds us!' shouted Myrddin.

The next charge came while the Emrys' words yet hung in the air – another slanting drive, the angle sharper this time. Again the Cymbrogi shouted to distract the horse, and again Llenlleawg broke off the attack – but carrying it closer before turning away.

'Llenlleawg!' the king cried. 'Here I am! Come to me!'

The champion galloped on, his face set, expressionless, his eyes staring and empty as the dead.

The third attack carried him almost headlong into the line. In the leaping firelight, I saw the head of the spear swing towards me as Llenlleawg began his charge. This time he came on a straight course and I knew he meant to break us. 'God help us,' I breathed, tightening my grip on Rhys next to me.

The black's hooves tore the turf as it gathered speed, legs churning, closing swiftly. I could already feel the spearhead slicing into my flesh and my bones breaking as I fell beneath those crushing hooves. I braced myself for the impact.

Llenlleawg charged to within a hairbreadth of the line. I could hear the spear blade sing in the air. But at the moment when the spear should have pierced my chest and carried me off my feet, the blade shifted and the horse blew past me – so close I could feel the heat of the animal as it surged by.

The line held, and the Cymbrogi cheered in their relief.

But when Llenlleawg did not so much as break stride, I knew that the testing was over. The next charge would be in earnest; the man chosen to meet it would die, and the circle would be breached.

Around and around rode Llenlleawg, straight-backed in the saddle, shoulders square, oblivious to the jeers and taunts of his former friends. On the final pass he began his charge. The horse strained forward, hooves pounding the earth. The spear came level as the horse turned onto its course, and I saw who had been chosen. The spear was aimed at Arthur.

'Hold, men!' he cried as the deadly blade swept swiftly nearer. 'Hold the line!'

The Cymbrogi, desperate to help their king, writhed in an agony of helplessness. Obedient unto death – each man willing, longing, to take the Pendragon's place in the line, yet unable to so much as lift a hand or move a step for the sake of that selfsame obedience – the brave Dragon Flight screamed their defiance at the onrushing traitor.

I could not bear to see the cruel spearhead pierce my lord and friend, neither could I look away. So, like all the others, I watched helplessly as the death-stroke hurtled swift to the mark. And, like all the others, I screamed in a futile attempt to draw the spear away from that mark.

Hooves flying, the black and its silent rider swept in.

The line tensed as if to meet the blow for the king. 'Stand firm!' shouted Arthur for the last time.

As Arthur cried out, the hard-charging horse stumbled, its forelegs buckling beneath it. The animal's speed and weight carried it forward, pitching the rider over its neck and onto the ground as the beast's hindquarters sailed up, back legs still kicking.

Llenlleawg fell headfirst to land sprawling on the ground. The spear struck the earth not two paces from Arthur's feet and buried itself deep, the shaft quivering with the force.

The line held, and we cheered our king's deliverance. Doubtless we would have swooped upon Llenlleawg if Myrddin had not prevented us. 'Peace!' he cried in his voice of command. 'Break not the circle, for the Great King upholds us still!'

Llenlleawg was on his feet again almost instantly. Up he leapt, hand on sword. As he drew the blade, I recognized it at once. How not? I have seen it every day for the past seven years. It was Caledvwlch, the Pendragon's own blade: the last evidence, if any were needed, of Llenlleawg's vile treachery.