He ran swiftly away and then returned, bearing in his outstretched hands a long silver blade with a handle encrusted with lapis and garnets. The priestess took it from him and stood over the lamb with her back towards us, holding the blade aloft and muttering incantations. I expected a longer ceremony and perhaps a series of questions, as many oracles required from their supplicants, and so I was a little startled when the blade suddenly-flashed and descended.

The priestess possessed skill, and more strength than I would have thought. The blade must have gone straight to the heart of the beast, killing it instantly. There were a few convulsions and a spattering of blood, but not a sound, not even the least whimper of protest as it gave up its life to the god. Would the slaves down in Baiae die as easily? In that moment a chill descended upon the place, though the air was still. Eco felt it as well, for I saw him shiver beside me.

The priestess slit open the lamb's underside from its breast to its belly, then reached inside. I saw how the hems of her sleeves had become so dark with bloodstains. She searched for a moment, then found what she was seeking. She turned toward us, bearing in her hands the lamb's quivering heart and a portion of its entrails. We followed her a short distance to the side of the temple, where a rude brazier had been hewn from the stone wall. The boy had already prepared the fire.

The priestess cast the organs upon the hot stone. There was a loud sizzling and a small explosion of steam. The vapour issued outward and then was sucked back toward the rock wall, drawn into fissures in the stone like smoke pulled into a flue. The priestess stirred the hissing entrails with a stick. The smell of seared flesh reminded me that we had neglected to eat at midday. My stomach growled. She cast a handful of something onto the heated stone, producing another cloud of smoke. A strange, aromatic scent like burning hemp filled the air, making me dizzy. Beside me, Eco swayed so violently that I reached to hold him up, but when I gripped his shoulder he looked at me oddly, as if it were I who had stumbled. I saw a movement from the corner of my eye and looked at the great wall of stone above and before us, where peculiar faces had begun to appear amid the fissures and shadows.

Such apparitions are not unknown at sacred shrines. I had witnessed them before. Still, there is always a sudden stirring of dread and doubt in that instant when the world changes and the powers of the unseen begin to manifest themselves.

Though I could not see her shadowed face, I knew that the priestess was watching me. She saw that I was ready. Again we followed her up a steep, stony path that traversed the slope, then descended into a dark, ever deepening ravine. The way seemed very far. The path was so difficult that I found myself stooped over, scrambling on my hands and feet. I glanced behind to see that Eco did the same. Strangely, the priestess was able to walk upright, striding forward with perfectly measured steps.

We came to the mouth of a cave. As we stepped inside, a cold, clammy wind rushed over my face, carrying a strange smell like the breath of many flowers in decay. I looked up to see that the cave was not a tunnel but a high, airy chamber, pierced all about by tiny holes and jagged fissures. These openings admitted a twilight glimmer, and the rush of the wind sighing through them created an ever changing cacophony that was sometimes like music, sometimes like a great chorus of moaning. Sometimes a singular sound would rise above all the others and then fade away – a trilling of notes like a satyr playing his pipes, or the bellowing voice of a famous actor I heard once as a boy, or the sigh that Bethesda makes before she wakes in the morning.

We descended deeper into the cave, to a place where the walls narrowed. The darkness deepened and the chorus of voices receded. The priestess raised her arm to signal that we should stop. In the dimness her blood-red robe had become jet black, so dark that it seemed to be a gaping hole that moved about in the grey gloom. She stepped onto a low shelf of stone, like a stage, and for a moment I thought that she danced. The black robe spun and twisted and seemed to fold in on itself. There was a long, wailing shriek that made my hair stand on end. The contortions were not a dance but the convulsions of the priestess as her body was possessed by the Sibyl.

The black robe fluttered to the ground, becoming nothing more than a great lump of cloth. Eco stepped forward to touch it, but I restrained him. In the next instant the robe began to fill again and rise up. Before our eyes the Sibyl of Cumae began to take shape. She seemed taller than the priestess, larger than life. She lifted her hands and pushed the cowl from her head.

Her face was barely discernible in the darkness, and yet it seemed that I could make out her features with a kind of supernatural clarity. I chided myself for ever imagining that the priestess was Iaia. This was the face of an old woman, to be sure, and in some superficial regards it resembled Iaia; the mouth might have been the same, and the high, gaunt cheekbones, and the proud forehead – but no mortal voice ever uttered such noises, and no mortal woman ever possessed such eyes, flashing as brightly as the light through the fissures in the cave.

She began to speak, then clutched herself. Her breast heaved, and a rattling sound issued from her throat as the god began to breathe through her. A sudden wind blew up from behind us and scattered her hair like flailing tendrils. She struggled, not yet submissive to the god and trying to shake him from her brain, like a horse trying to unseat its rider. Her mouth foamed. Noises came from her throat like wind in a cavern, and then like the gurgling of water in a pipe. Little by little the god mastered her and then calmed her. She hid her face in her hands, then slowly drew herself erect.

'The god is with me,' she said, in a voice that was neither male nor female. The Sibyl seemed merely to mouth words that issued from some other source. I glanced at Eco. His forehead was beaded with sweat, his eyes were wide open, his nostrils were dilated. I clutched his hand to give him strength in the darkness.

'Why do you come?' the Sibyl asked.

I started to speak, but my throat was too thick. I swallowed and tried again. 'We were told… to come.' Even my own voice sounded unnatural to my ears.

'What do you seek?'

'We come… seeking knowledge… of certain events… in Baiae.'

She nodded. 'You come from the house of the dead man, Lucius Licinius.' 'Yes.'

'You seek the answer to a riddle.'

'We seek to know how he died… and by whose hand.' 'Not by the hand of those who stand accused,' said the Sibyl emphatically.

'And yet I have no proof of that. Unless I can show who murdered Licinius… every slave in the household will be put to death. The man who seeks to do this thinks only of his own advancement… not of justice. It will be a cruel tragedy. Can you tell me the name of the man who killed Licinius?'

The Sibyl was silent.

'Can you show me his face in a dream?'

The Sibyl set her eyes upon me. An icy shiver ran through my bones. She shook her head.

'But this is what I must know,' I protested. 'This is the knowledge I seek.'

Again the Sibyl shook her head. 'If a general came to me and asked me to strike his enemies dead, would I not refuse? If a physician came and asked me to heal his patient, would I not send him away? The oracle does not exist to do the work of men for them. Yet if these men came to me seeking only knowledge, I would give it. If it were the will of the god, I would tell the general where his hidden enemy lurked, and I would tell the doctor where he might find the herb that could save his patient. The rest would be up to them.