We inched onwards. I felt the stone of the arch brush the length of my spine. I kept the Glock’s barrel half an arm’s length in front of me and felt above my head with my left hand. Stone. And then void. I swept the same hand down and left, making contact with Baaz’s right arm. I held it fast and took another step.

‘Listen,’ he said, almost inaudibly.

But I could hear nothing. It was the most profound silence I’d ever experienced. Quieter than the jungle before dawn; quieter than the caves in Sierra Leone; quieter than the lake water that lapped at the edges of my dreams. We stood for a few seconds like that, me clutching his arm, engulfed in nothingness.

‘Don’t shoot,’ he said at last. ‘Just look.’ And with that he flicked on the torch. I lurched forward, finger tensed on the trigger. But no shot sounded. We were completely surrounded – encircled by hundreds, thousands of people. But Baaz was right: there was no one to shoot at.

Skeletons piled up around us. Femurs and tibias; humeri and ribs; and row upon row of uncounted skulls stacked high. The chamber must have been twenty metres across, but so densely packed with bones that the circle clear enough to stand in was less than five metres wide. Baaz looked at me, for my reaction, and then played the LED slowly across the ancient faces staring at us.

Here we were, fighting for our lives, standing in the centre of a mass grave. For an instant I thought of the dead I’d left behind me, the piles of bodies that marked the waypoints in my journey from orphan to assassin. How much space would they take up? I didn’t know. I didn’t want to know. The only thing I knew for sure was that I wasn’t ready to join them. Not yet.

‘Just for kicks,’ he said. ‘If you come in with the light on, it’s not the same.’

I said nothing. The shadows cast by the blue-white beam had the curious effect of making the mandibles of the skulls appear to work, grinding jaws which had not tasted meat for centuries. Coated with fur and mould and cavern slime, long bones stuck through smashed pelves. Finger joints crunched underfoot like pea gravel on the Devil’s driveway. Cave-slime ringed the eye sockets – glistening emerald irises winking back in the torchlight.

I spun around the illuminated epicentre of that empire of the dead, avoiding their gaze, searching for the exit Baaz had promised.

‘Here it is,’ he said, lighting up the far corner. ‘I told you. It’s crazy, isn’t it?’ He’d lit the tiniest of apertures, a one-foot by two-foot opening where the sweep of decaying bones curved up to meet the ceiling. ‘No one has ever been up it,’ he said.

And I could see why.