A plain altar carved from a large piece of granite occupied the central ground before the shrine itself, its surface covered in disquieting red-brown stains that no rain could wash away. Fabiola did not let her gaze linger on the stone slab, moving it to the carved columns that held up the triangular decorated portico. They were shorter and less grand than those of many other shrines, while the steps up to the entrance had not been cleaned in an age. Yet the depictions of demons and evil spirits sprang out from the faded paint above. There were sharp horns, probing tongues, mouths full of sharp teeth and outlandish weapons galore. Fabiola recognised Charon, the blue-skinned Etruscan demon of death, with his feathered wings and massive hammer. At gladiatorial games with Brutus, she had witnessed a living man play Charon's part, entering the arena to mock screams from the audience. There his role was real, and gruesome. The memory of his hammer smashing the skulls of the fallen to ensure that they were dead still revolted Fabiola.

The figure over their heads looked fully capable of the same, but Charon paled into insignificance beside the painted representation of Orcus himself. Occupying the central part of the triangular portico, the god's stern, bearded face was enormous, with a diameter at least twice the length of an ox cart. His dark eyes stared down fiercely, transfixing Fabiola. She could not bring herself to look at Orcus' hair, which was a writhing mass of snakes. Ever since another prostitute had placed a venomous serpent in her bed, she had been terrified of the creatures.

She jumped as Sextus touched her elbow. 'Let's get inside, Mistress,' he urged. 'This rain will give us a fever.'

There was no point holding back now. Praying that her plan would not backfire, Fabiola climbed up the steps to the entrance, followed closely by her slave. Past the rows of fluted columns were two tall doors, their surfaces covered with strengthening iron strips. They were shut, and Fabiola quailed. Was Cerberus waiting to devour her on the other side? Come on, she thought angrily. I am alive, not dead. Rallying her courage, Fabiola stepped up to the portals and thumped on the wood with a balled fist.

Apart from the rain drumming off the ground behind them, there was silence.

She banged harder this time. 'Open up! I wish to make an offering.'

A long pause followed, and Fabiola scowled. There were definitely people inside, she knew that. A temple complex such as this was no different to any other in Rome: it was where the priests and acolytes lived, ate, slept and worshipped. Apart from occasional sacred days – and today was not one – they were open to the public every day of the year. She raised her hand again, but as it fell, the door was pulled silently ajar. Startled, Fabiola lowered her arm and took a step backwards.

A grey-robed priestess stood framed in the entrance. She was young, perhaps the same age as Fabiola. Short, with long brown hair pinned up behind her head, she had a wide face with a short nose. Piercing green eyes studied Fabiola, disconcerting her.

'Enter.' She moved aside.

Fabiola was reminded of someone, but was so wound up that she gave it no further thought. Pushing back the hood of her cloak, she crossed the threshold with a mental prayer to Mithras for his protection. Fabiola felt no qualms about this; it was not unusual to ask things of many gods.

The corridor within ran from side to side away from the doors and was even dimmer than the street. Occasional small oil lamps hung from brackets, casting long, flickering shadows on a bare, stone-flagged floor. Grotesque paintings of gods and demons covered the walls, their limbs cleverly moving in the guttering light cast by the lamps. The threatening atmosphere was a deliberate construct, Fabiola realised, generating anxiety in visitors' hearts the instant they set foot inside. Yet this was the temple of Orcus, the god of the underworld. It was right to be scared here. Despite herself, Fabiola shivered. Do not forget your purpose, she thought, shoving down her rising dread. 'I wish to make a request of the god. In private,' she said, opening her clenched fingers. On her palm lay three neatly folded pieces of lead. She had spent hours composing the curses inscribed within them. With the threat from Scaevola more immediate, all referred to him, requesting his death in the most terrible of ways. For now, Caesar came second.

The priestess was unsurprised. People came here for every reason under the sun: twisted with hatred, seeking retribution for wrongs done to them, asking for revenge on enemies, lovers and superiors. Extreme weather did not remove such needs, nor did it affect the desire of certain devotees not to be seen by others. 'Follow me.' She walked off, her bare feet slapping off the floor.

Nervously, Fabiola and Sextus followed. In silence, they passed a succession of doors, all of which were closed. Fabiola wondered who might be in the chambers beyond. From one came the low sound of men chanting. She couldn't make out the words, but the tune was slow and mournful and did little to calm her jangling nerves.

The priestess came to a halt at last. Producing a key from within her robes, she unlocked the door before them, which opened noiselessly, adding to the air of pressure. Inside was a large windowless chamber, its plastered surfaces painted an ominous, dark red colour. As in the hallway, the only light came from a few oil-burning lamps on the walls. There was barely any furniture, apart from a plain cement furnace on a square platform of bricks, situated at the back of the room. Staring in, Fabiola felt a warm current of air bathing her cheeks. A strong smell of incense also carried through the doorway. A deep red glow in the oven's opening revealed the source of the intense heat. To one side of it lay a pile of fuel, and on the other sat a small altar decorated with a statue of Orcus.

'You may make your offering here,' said the young priestess. 'Without interruption.'

Fabiola's grip on the lead squares grew so tight that she felt them begin to bend at the edges. She stopped, worried that any damage might affect her requests of the god. Nothing must go wrong. Her very life depended on it. Nodding firmly, Fabiola walked in, tailed by Sextus.

The priestess also entered, shutting the door. Moving to the altar, she bent her head in prayer. Unsure what to do next, Fabiola did the same. Compared to the cool of the corridor and the rain-soaked streets, this room was like a caldarium, the hottest place in a bathing complex. Thanks to the incense which was burning, the atmosphere was heavy and intense. Despite her soaked clothing, Fabiola felt sweat break out all over her body. She was used to the fuggy warmth of a full Mithraeum, but this was different. Some temples had fires to throw small offerings on, but not this roaring furnace, which reminded Fabiola of what Hades might be like. Fresh fear gripped her, yet she forced herself to stay calm. Orcus was no ordinary god. Gifts to him were cast in their entirety into the flames, there to be consumed. Hence the need for the oven.

Orcus, Fabiola thought, raising her eyes to the statue. Implacable, it stared right back. Mighty god of the underworld, hear me, she entreated. Once again, my life is in danger from Scaevola. He is an evil man and a murderer who will stop at nothing. I have no real means of stopping him without your help. Rid me of this whoreson, and I'll be in your debt for ever. I will erect an altar to you, and there a goat will be sacrificed once a year for the rest of my days. As an extra incentive, Fabiola leaned forward and placed a stack of silver coins before the figurine. A sharp intake of breath from the priestess proved that the amount was impressive.

There was a loud crackling sound and flames belched up inside the furnace. Startled, Fabiola craned her head to see. Neither Sextus nor the priestess had done anything, but the fire was now roaring as if a smith was working a pair of bellows on it. She looked around, expecting to spot a demon hard at work, but all she could see were the four crimson walls, pressing in on her like a tomb. Long yellow-orange flames licked at the oven's opening, making it seem like the glowing maw of a ravening mythical beast. Terror overcame Fabiola at last and she froze.