‘Whose pet?’ I asked.

‘Don’t know. Like a dream. Pet and bright water. And a great blank something.’

He was most impressive there in the dark. I refrained from asking him whether the pet was Arthur’s and lived in the cauldron. And bright water was a pleasant dream when there was a chance that we both might finish up in black.

The first we heard of them was a dragging of timber, and we hurried down the shaft. The major entered his private recess and returned to his mattress. I ran on, crossing the pool and, when I reached the far side, taking care to shake the drips off shoes and trousers over the edge so that no fresh sparkle of water should give away my passage. For the rest of the route there were enough old footprints for my own to be muddled.

I found that the stream, after it flowed out of the side wall of the cavern and on to the lake, was shallow and ran in a bed worn down about a couple of feet below the level of the floor. In wading it I was only wet below the knees, but to enter the low mouth I had to crouch down and accept a soaking. The water, however, was not so cold as I expected, possibly descending from the divine warmth of midsummer rills down to the realm of Gwyn ap Nudd. Inside the channel Gwyn came into his own. It was riven and irregular, some of the rocks smooth with deposit and suggesting the rounded backsides of burrowing beasts, some jagged and fallen or seeming about to fall from roof and sides. Round a dark corner the passage became high and narrow. I took station just short of it, ready to retire into the cleft before a searching beam could reach me.

The party were long in arriving and had left Evans behind. There were six of them, all in white robes and each carrying a red and smoky torch: the three master druids – if that’s what they called themselves – whose names I knew, plus the others who had been at the forest ceremony. One of these looked thoroughly dangerous, like a heavyweight boxer who had been converted and found salvation.

The major was unaccountably absent. A good sign, I thought. They had got no more out of him, and he had no business at the hallelujah party. On the other hand, why was there a party at all since they had only come down to find out how and by whom the entrance had been opened? Very soon I had proof that they were not sure the major had told the truth. Ballard walked over to check the mouth of the stream. He didn’t like it. He took the cold-water treatment like a man, but he was going no further. That hole, once you were beyond the entrance, was a fitting home for the nastier spirits of the underworld. He was perfectly right to be content with shining a light inside. At the corner was waiting the werewolf, but it lay flat behind a boulder and had no intention of appearing till stepped on.

Ballard returned to his fellows and I to the mouth of the channel. For a moment there was such a silence that the faint ripple of the stream seemed to echo back from the walls of their cathedral. Then the dark tunnel of the approach road was faintly illuminated, and the congregation lined itself up, three on one side of the altar and three on the other.

Evans made his dramatic entry in Marrin’s blue robe, but without his dignity. He was leading the major by a light chain looped round his neck. Denzil seemed submissive, as if he were an animal destined for sacrifice, though whether because he was busy accepting martyrdom or whether any tug on the chain was painful I could not tell. Having arrived in front of the altar, Evans placed him at the centre of the circle, lifted the chain and substituted a wreath of yew. I never admired Denzil more. He took this sinister mummery as impassively as a recruit accepting a slight adjustment of the helmet. His eyes looked proudly into a past known only to him.

On each side of Evans and the major was a line of three, holding their torches so that the whole scene was enveloped in a thin red mist. All eyes were on Evans and the altar. Though one line faced me, I thought I could take a risk in the prayerful concentration of hocus-pocus. I slipped out into the bed of the stream and crawled down it, hidden by the edge of the cavern floor, until I reached the lake. It was quite shallow where it met the rock and I continued crawling, very gently without making any wash, up to a point where I was below and behind the altar. If Evans went round it or leaned across it he was bound to see me, but so long as he was officiating I was safe.

I had the impression that these raised arms and murmured prayers were a preliminary to something more serious. Preliminary to what? Human sacrifice of course went through my head, but I couldn’t believe it. The object of all this was more probably to dedicate the major to the divinity of the altar and so involve him in the mysteries that his religion rather than his body was sacrificed. If so, they underrated their man They knew that he had a limited sympathy for Broom Lodge faith, though distrusting Marrin and very far from accepting membership. They were also aware that he did not wholly reject – and that’s putting it mildly – the conception that the golden cauldron, whether found or materialised by Marrin, could be the Grail. What they did not realise was that he possessed two impregnable fortresses of resistance: that of the trained, sane, devoted Guards officer and a spiritual courage worthy of the Guardian of the Grail.

The prayer meeting broke up. Time passed. Nothing happened. I hugged the sheer bank of the lake below the altar, in deeper shadow now that the torches had moved away. Evans was standing by the windlass erect and silent, his right hand resting on the tail of the dredge, his left napoleonically thrust into his robe. A group of three were strolling and muttering like monks in a cloister. Two others were away towards the far end of their temple. The seventh was near the junction of stream and lake, and if he had looked to his left he must have seen me or at least a lump where there shouldn’t be a lump. Fortunately he walked towards the dark mouth of the stream and stared into it, doubting, I think, if Ballard had done a proper job of exploration.

Still nothing happened. The absence of tension was itself nerve-racking because one dreaded what the climax of the service would be when it was resumed. They might be waiting for midnight or some other propitious hour, or possibly for some sign from the major himself. I was again reminded of Mithraic ritual. In that case Denzil was being subjected to the Trial of Fortitude. Would he break down and scream because nobody was paying any attention to him? Would he rush out of the circle and attack the nearest officiant? He stood quite still within the spiritual wall. To the believers the apparent hypnotism may have been impressive magic, but in fact there was no magic about it. Denzil could gain nothing by moving. Escape, one against seven, was impossible. Meanwhile, among the unbounded number of places where he might choose to stand, the circle was the most obvious and most dignified.

Time went on and on until I found that my own fortitude was being severely tested. To lie, cold and motionless as a corpse in the black lake, to do nothing, absolutely nothing, while waiting to observe blood sacrifice or physical torture or the madness of hallucination was torment, and I doubt if I would have had patience if I had not appreciated that all this seemingly casual idleness was a most effective technique not only for softening up the victim but for provoking the observer – if there was one – into giving his presence away.

I heard a sort of collective gasp and put my head over the edge. Into the scattered lights of the cavern something advanced down the path. Above it was the shape of the cauldron picked out by red reflections and seeming to be sailing at head height down the tunnel by itself. Everyone of the coven was motionless, turned towards it. The major made his first movement. He knelt down.