‘You’ve done well,’ she once remarked. ‘What you did in Capri, Parmenon, was more than enough. I will keep you like an arrow in my quiver. Hidden from public gaze until the time comes. .’

Early in the spring of Tiberius’s last year, just after a Roman force had been ambushed and cut to pieces by the Frisians on the empire’s northern border and the survivors cruelly tortured, Agrippina dropped her mask.

At first I thought it was the break in the weather: a sudden, glorious spring had transformed Rome in bursts of golden sunlight, fresh flowers, and sweetness in the air. The blood bath had begun to abate; the list of proscriptions appearing less often in the Forum as Tiberius became more engrossed with the security of Rome’s frontiers. The change in Agrippina began almost imperceptibly, but once I had noticed it, I realised that her spirit had revived. She met me early one afternoon, in one of those flowery grottoes so lovingly designed by imperial gardeners. She’d been reading poetry again and talked about visiting the theatre. She looked round like a young girl ready for mischief, placed her reading tray on the turf seat beside her, got up and put her arms round my neck.

‘Parmenon, I am pregnant!’

She seemed so excited, so full of life. I tried to hide my jealousy. ‘Which means,’ I replied sourly, ‘Tiberius must be dying.’

This was one of those bitter remarks which slip from your tongue before you can think. It was also highly dangerous. On any other occasion Agrippina would have been angry but today she drew me closer, those dark eyes bubbling with laughter.

‘Parmenon, you should be an astrologer. The monster is dying!’

Now I was nervous. I pulled away and went to the edge of the arbour and stared round the garden. Agrippina was always cunning: there was no one about.

‘You don’t believe me do you?’ she asked. ‘Parmenon, the old cadaver is dying at last. I doubt if he’ll last till summer.’

‘How do you know this?’

‘Oh I do. But you have fresh orders: you’re to visit Tiberius.’

‘Orders?’ I demanded. ‘Is that what I am now, Domina, your lackey?’

‘Parmenon, Parmenon.’ She held her stomach. ‘Believe me, if I thought Tiberius would live, I would never have conceived.’

‘How?’ I whispered.

‘Oh the usual way,’ she laughed. ‘Domitius got into my bed and I gave him free rein.’

‘No,’ I contradicted. ‘Tiberius?’

‘The queen of all poisons, Parmenon. Aconite.’

I stared in disbelief. Agrippina, even then, had an interest in the collection and mixing of poisons. Aconite, ground from herbs grown in the Alps, was rare, powerful and went under many names: wolf’s-bane, woman’s-bane. Its effects were very much like dropsy: alternating bouts of heat and chill, numbness of the limbs, tingling in the mouth and throat, giddiness, loss of feeling.

‘Caligula?’ I asked.

‘He’s quite the gardener.’ Agrippina could hardly stop laughing. ‘You know how Tiberius likes his vegetables? Well, he encouraged Gaius to be a gardener and my darling brother faithfully followed the Emperor’s orders. “Little Boots” has got more poisons than I have in my cupboard. Remember the message you took to Caligula about his salvation being on Capri? It was a reference to Aconite specially grown there. Gaius has been killing Tiberius drop by drop, mixing it with his snails and cream, and a little with his wine.’ She giggled behind her fingers. ‘Sometimes it was even mixed in with dishes of horseradish and we all know how much Tiberius likes his radishes!’

‘He might be caught?’

‘Caught? Someone else will take the blame.’

‘I thought Tiberius took antidotes to all poisons?’

‘He does. He’s a regular medicine chest. You’ve heard the phrase, “creaking doors hang longer”. Tiberius has so many ailments he doesn’t really know what’s happening.’ She stepped closer. ‘Aconite, if fed long enough, will overcome any antidote. Gaius has simply increased the usual dosage.’

‘And why now?’ I asked, looking furtively over my shoulder.

‘Oh, don’t be so nervous, Parmenon. Macro knows everything, and this garden is guarded by his best men.’

‘Why now?’ I repeated.

Her face became grave. ‘If Tiberius lives, Parmenon, he’ll take all our necks. Can’t you see that? In the years that have passed since Sejanus’s death, Tiberius has started to reflect and regret, wondering whether Macro was part of a more sinister plot. With my mother and two of my brothers dead only my two sisters and I are left, apart from Gaius. Tiberius might decide to make a clean sweep.’

She plucked a parchment from beneath her robe and handed it to me. I undid it carefully: the writing was scrawled like that of a child.

‘Tiberius was never good with a pen,’ Agrippina declared.

The very first word chilled me: Proscripti — the Proscribed. Tiberius spent his time drawing up lists of those he would like to see strangled. The ink on this list was faded.

‘It was written some months ago,’ Agrippina explained.

The list was long. Some of the men and women on it had already died. After a gap, some new names appeared: Agrippina, her two sisters, Drusilla and Julia, Macro, and Parmenon. I thrust the parchment back.

‘When do I leave?’ I demanded.

‘As soon as possible. At the moment Tiberius is in Campania, roaming up and down the countryside, too frightened to enter Rome, but he intends to sail for Capri very soon. Once he gets back there, he’ll return to his death lists. Macro has prepared well for this. He’s already been in touch with the generals on the frontiers and their loyalty is assured.’

She went and sat on the seat, crossed her legs and leaned back, eyes half closed, gently rubbing her stomach.

‘Tiberius must not return to Capri. He already thinks he’s going to die, he’s been having visions. A new statue of Apollo, which is to be erected in the Senate library, apparently appeared to him in a dream and said: “Tiberius, you will never dedicate me.” There have been fires that are suddenly extinguished or abruptly flare up. Tiberius’s pet snake, which he fed with his own hands and was the only creature which would come anywhere near him, abruptly died and was eaten by a swarm of ants.’ She leaned forward. ‘Caligula would have enjoyed arranging that. I must ask him how he did it. Tiberius is agitated; he left Capri and came up the Tiber in a trireme, posting troops along both banks so that no one could see him. He’s presently at Misenum: Caligula and Macro are there with him.’

‘What am I to do?’

‘Make sure that Tiberius does die,’ she whispered. ‘And when he does, return immediately to Rome.’

I reached Misenum late the following evening, and it was apparent how grave matters had become. Cavalry units and squads of Praetorians patrolled every path to and from the imperial villa. The house itself was like a mausoleum with everyone tiptoeing around. Macro and Caligula met me in the gardens. The soldier was the same as ever, cynical and mocking, but Caligula had changed. He was now taller, but more stooped, his head balding, his eyes deep-set, a cynical sneer twisted the cruel mouth, and his arms were covered in thick, black hair. He reminded me of an ape I’d once seen in the arena. He was drinking deeply and couldn’t sit still, walking up and down the path, staring up at the dark-blue sky. Occasionally, he’d turn away, muttering and talking to someone we couldn’t see.

‘What’s the matter with him?’ I whispered.

Macro put a finger to his lips and led me away.

‘He’s talking to his Daemon, which you must never ask him about. He’s drunk, tired and agitated. He still believes Tiberius will order his execution!’