'Either we keep riding until we find them, or we go back,' I suggested, and Rhys rolled his eyes to show how impressed he was with my reckoning. 'Which is it to be?'

Before he could answer, there arose on the path behind us the strangest sound I have ever heard. If you were to imagine the sound of a bull stag belling out his rage as a pack of baying hounds raced in for the kill – imagine that, and then increase it tenfold and add to it the roar of a stream in full spate, and you will have some small idea of the sound that broke upon us as a single blast, like that loosed from a horn.

A seething, restless silence reclaimed the trail. The horses shied and tried to bolt, but we held them tight. In a few moments, the sound came again, closer. The bare-limbed trees quivered, and I felt the dull tremble of the earth in the pit of my stomach. Whatever made that sound was coming our way, and swiftly.

The next sound I heard was the sharp slap of leather against the withers of Rhys' mount as he wheeled the frightened animal and gave it leave to fly – nor was I slow to follow, pausing only long enough to cast a fleeting backward glance. I saw nothing but the shadow-crowded path and darkness beyond. Even so, the shudder of the trees told me that the thing was charging towards us with speed.

I gave my mount its head, and a heartbeat later, I was racing along the forest trail, trying to catch Rhys.

It took us longer to reach our waiting companions than I expected, and I feared we had somehow lost king and Cymbrogi along with all the rest. But then Rhys slowed and I saw, just beyond him, two horses in the track ahead. The Cymbrogi had dismounted to rest the horses while awaiting our return. They called out to us, asking what we had discovered, but we did not stop until we had rejoined Myrddin and Arthur.

Rhys slid from the saddle before his horse had come to a halt. Arthur and Myrddin had risen to their feet, the question already on their faces. 'We did not find them, lord,' Rhys was saying as I dismounted.

'Then what -' began the king.

Before he could say more, the creature behind us loosed its bone-rattling cry. The forest trembled around us and the horses began rearing and neighing. The waiting warriors leapt to their mounts, stretched for their dangling reins, and retrieved spears from beneath their saddles.

Arthur, sword in hand, ordered the battle line and, an instant later, we were armed and ready to face whatever came our way.

The trail was too narrow for horses to manoeuvre, so Arthur ordered the fight on foot. 'It will come at us on the trail,' the king cried, his voice taking on the vigour of command. 'Let it come! Open a way before it – make a path – two men on each side. Let it come in – then close on it from either side.'

It was a desperate tactic, borrowed from the hunt, most often used when a man finds himself unhorsed during the chase. Arthur established himself at the forefront of the line. Myrddin stood to his right, with Rhys and me to his left. The Cymbrogi led the horses to safety well up the trail, and then quickly filled in behind us in ranks four across.

We stared into the gloom, tree limbs quivering on either side and overhead. I could feel the trembling of the ground as the shudders passed up through the earth and into my feet and legs. A hundred horses pounding hard down the path could not drum the earth so. What could it be?

The unnerving cry thundered again. Closer. The entire forest seemed to ripple like a wave. The unnatural sound sent a cold flash of fear snaking through the ranks.

The drumming thud in the ground grew louder. The Cymbrogi stood gripping their spears in silence, staring hard into the gloom ahead.

The roar sounded again. Closer still: an unearthly howl that pierced to the heart. Cold, sick dread spread through me and the wood seemed to undulate; a black mist gathered before my eyes as the ground shook with the pounding of unseen hooves.

I tightened my grip on my spear and shook my head to clear it, thinking, The thing must be nearly on top of us now… but where is it?

And then I saw, looming out of the murk of shadows, the form of a beast: a great dark mass racing with impossible speed directly towards us. God help us, it was enormous!

Out of the shadows it came. I heard several stifled cries behind me, and others gasped and muttered hasty prayers.

Curiously, the creature had no substance, no solidity. Even as it swept swiftly nearer, I could get no clear notion of its appearance. The thing seemed nothing but shadow and motion. Indeed, I could see the dimly quivering shapes of trees and branches through it.

The ground heaved beneath our feet, and I smelled the rank scent of animal filth. But though we stood steadfast with our spears at the ready, there was nothing substantial to fight.

I received the distinct impression of a massive beast with the sharp spine and high-humped shoulder of a boar, its foul hair long and flowing in matted shreds like the tatters of a rotten cloak. I imagined two huge yellow eyes glaring balefully out from a flattened piglike face, beneath which bulged a massive jaw from which two great, curved brown tusks jutted in upward-sweeping arcs like a pair of barley scythes. Short, powerful, stumplike legs pummelled the earth, driving the creature forward on the cloven hooves of a stag.

This, as I say, was merely an impression, an image that burned itself into my mind. There was no actual creature, nothing corporeal at all – just a dark-gathered mist of churning shadow and motion.

Some of the warriors let slip their weapons, and one or two dropped to their knees.

'Courage!' shouted Arthur, his voice a steady rock amidst the rising flood of fear. 'Stand firm!'

The vile thing drove down on us with the speed of a falling mountain, shaking the ground with every flying step. I gripped my spear and hunkered down, ready to let fly should anything tangible present itself.

The beast came on. The monster loosed its earsplitting scream. The chill air shivered to the sound of a thousand slavering hounds and the belling of a hundred stags at bay.

The cry carried the shadowshape into our midst.

'Hold!' called Arthur. 'Hold, men… stand your ground.'

Beneath my feet, the ground rumbled hollow like a drum., 'Stand firm…' Arthur called, straining to be heard above the sound of the onrushing beast. 'Stand…'

My stomach tightened in anticipation of the terrible impact. The air shuddered and I had the explicit sensation of a great hairy flank heaving past me – like a rippling black wall of muscle.

Spear poised, I drew back my arm and prepared to strike.

The warrior opposite me let fly – too soon! The spear sailed over my head; I ducked under it and in the same instant heard a short, sharp cry as the creature whirled in mid-flight and struck.

I saw merely a sudden surge, a quickening of the darkness, and the monster thundered past.

I leapt to the stricken warrior's aid, and a stink like that of rotting meat struck me like the blow of a fist. The gorge rose in my throat and I gagged on the stench. I put a hand over my nose and mouth to keep from vomiting. The Cymbrogi round about groaned, coughed, and spat, and the wounded man writhed on the ground.

His side had been laid open from chest to hip, and blood gushed dark and hot from the wound. 'Help me!' he screamed. 'Help me!'

'Tallaght?' I said. In the dim light, his features twisted with pain, I did not recognize him at first. 'Lie still, brother. Help is coming.

'Myrddin!' I shouted. 'Over here! Hurry!'

Tallaght clutched my hand; his grip was slippery with blood, but he clung to me as if to life itself. 'I am sorry, lord,' he said, his voice already growing weaker. 'I did not mean to disgrace…'