Claudius’s mother, Antonia, was a different dish of onions: sharp and shrewd, upright in her life, she knew the true Caligula. One evening though she berated him too openly at a banquet, and Caligula lost his temper. He cleared the room and leant over his aged aunt, his face a mask of hatred. ‘I can do what I like!’ he hissed. ‘I’m the Emperor!’

Agrippina intervened, pulling her brother away. A short while later Antonia died, some say from poison. Caligula didn’t attend the funeral rites. Instead he feasted and banqueted and watched the pyre burn, all the time murmuring, ‘I can do whatever I like! I can do whatever I like!’

Indeed, he did do what he liked. Some of his gestures were noble. A slave woman, who had been racked and tortured by Sejanus but refused to betray her master was given her freedom and the huge sum of 800,000 sesterces. On another occasion I was with Caligula when he visited the public baths — he’d taken a liking to me since Tiberius’s death — accompanied by a small retinue of slaves, one of whom carried his linen towels, flask of oil, perfume jar and strigoil. On entering the baths Caligula saw an old man, an ex-soldier, rubbing his back up against the wall, and told me to call the old soldier over.

‘I can’t afford a slave to clean my back,’ the old man explained. ‘So, rubbing it against the wall is the best I can do.’

Caligula immediately gave him two of his own slaves as a present. The next time he returned to the baths, at least three dozen veterans were rubbing their backs up against the wall. Caligula roared with laughter and told them to clean each other’s back: the Emperor’s wit became the toast of Rome. All seemed well. Agrippina now withdrew to her own house. She hired every physician in the city to advise her on what to eat and what to drink, how best to protect the child growing within her. When she wasn’t talking to them, she was closeted with me. She’d question me closely about Caligula’s moods, advise me what to say, what to do, and warned me to watch for certain symptoms.

‘What do you fear will happen?’ I asked.

‘The same as if a hungry panther burst into a chamber,’ she replied.

The panther sprang in the September following Tiberius’s death. Caligula was now obsessed with the games and staged a massive spectacle for the citizens of Rome. I rose early, in the still time just after cock crow. I had a light meal of bread soaked in watered wine and prepared Domina’s litter and retinue, to take us to the games. Every merchant and food-seller in Rome had flocked to swarm round the gates selling honeyed cakes, spiced sausage, lizard fish and boiled eggs, and the air was sweet with spices and perfumes. I joined Domina in the imperial box overlooking the amphitheatre. Caligula was there with his new plaything, whose name I forget, though as usual the place of honour was reserved for Drusilla, accompanied by her weak-jawed husband Lepidus. We sat behind them, but Agrippina moved so she could watch her brother’s face. Caligula was highly excited.

The games started with a mime featuring a dog. The animal was given food which, the audience was informed, was laced with poison. The dog ate it, exhibited hideous convulsions and fell down, to all appearances dead. He was picked up by his master, carried around and laid down on a mock funeral pyre. The man clapped his hands and the dog sprang to his feet. Caligula was beside himself with laughter. He grabbed a purse from Agrippina and threw it into the amphitheatre. He turned round, his face only a few inches from his sister, flushed, his eyes bright and starting.

‘Find the name of that poison, Agrippina,’ he whispered hoarsely. ‘And I’ll never use it again!’ He jumped up and down clapping his hands.

‘The time of the panther,’ Agrippina whispered. ‘But we’ve done what we can.’

Caligula loudly applauded the tightrope-walking elephants and those other animals which had been dressed up in male and female costume, but then his mood changed abruptly.

‘Iugula! Iugula!’ he screamed. ‘Cut his throat! Cut his throat!’ The cry of the mob when the gladiator was down.

The games manager recognised his cue for the carnage to begin. Gladiators poured into the arena to a fanfare of music. The combatants whipped themselves up into a fury by shouting abuse at one another. The music grew more raucous; trumpets, horns, flutes brayed and shrieked and the real bloodshed began. Caligula was beside himself, screaming abuse. As the sun grew hotter, the crowd began to demand a break for their usual refreshments. Caligula, getting to his feet, bawled at them to shut up. He had the awnings removed so the mob would learn its lesson and suffer the full brunt of the burning sun. Agrippina hastily called a servant and poured a goblet of wine — only I saw her add the powder — which she thrust into her brother’s hand. Meanwhile, in the arena below, the red-gold sand was littered with corpses. The gladiators, who had fought for hours, were now looking askance at the imperial box, where Caligula had dozed off. Agrippina whispered to the games manager. She ordered the awnings replaced and water and cakes to be distributed to the crowd.

Caligula slept for an hour. When he woke, he was paler, and more composed, taking more interest in Drusilla than in the tally of mounting corpses for the rest of the afternoon. The games finished and he returned to the imperial house on the Palatine, where he had ordered a banquet that was to cost the treasury millions of sesterces. Caligula demanded that Drusilla share his couch, where he lolled, drinking incessantly, whilst every possible dish was served: young kid, pheasant and goose, lamprey and turbot, sow’s udders. All the best chefs in Rome had been hired for the occasion, and Caligula led many of the guests on a tour of the kitchens, where they flocked like starving, screeching peacocks, standing on tiptoe, biting their fingernails as they watched each chef prepare a dish. The Emperor had insisted on inviting the ‘Victor Ludorum’ from the amphitheatre, a burly Thracian with the nickname of ‘Lord of the Dolls’ because of his sexual prowess amongst the women of Rome.

Caligula was beside himself with pleasure at the consequent revelry and chaos. Musicians and jugglers noisily thronged about and, despite Agrippina’s efforts, Caligula joined them. He insisted that the jugglers explain how objects thrown into the air seemed to fly back into their hands. Agrippina ate and drank nothing. She tried to distract her brother with comedians and actors who performed a bedroom farce, ‘Love Locked Out’, which Caligula watched intently. After the first act, he kissed Drusilla full on the lips and staggered to his feet, before stopping convulsively as if poisoned, staring at an actor wearing a bright red mask. Caligula thrust a hand out towards him.

‘So, you are back!’ he bellowed. ‘Has Tiberius sent you up from Hell? Who invited you here?’

Agrippina half rose from her couch. The Emperor’s screams stilled all the clamour.

‘What are you doing here?’ Caligula demanded.

The actor wearing the mask stood rooted to the spot. Caligula’s hand flew up in the air, he gave a loud scream and collapsed to the floor. Agrippina and Macro immediately took charge: the banqueting hall was cleared and the Emperor was hastily carried to his bedchamber, to which physicians, including Charicles, were summoned. As Agrippina supervised their ministrations, I heard her whisper, ‘He can’t die, not yet!’

Caligula was as white as a sheet, his breath coming in short dying gasps. Indeed, at one point Agrippina had to hold a mirror to his mouth to see if he was still breathing. Charicles examined his mouth and eyes and took his pulse before coming in to the antechamber, where Agrippina, Macro and I had gathered, together with that sly-eyed Greek, Progeones. The inclusion of Progeones was Agrippina’s idea. He was one of those hybrid creatures, who seemed neither truly male nor female. Oh, he had a man’s face and testicles, but the way he talked and moved, especially the flutter of his eyelids, the turn of his mouth and his high-pitched tone, were more suggestive of a woman. He walked with a better wiggle than some of the best courtesans in Rome. Agrippina had hired him to keep an eye on Caligula when he went visiting in the pleasure houses and brothels of Rome. She trusted him completely, claiming to have enough proof of that creature’s execrable habits to have him crucified or strangled a hundred times. I hated him, with his curling eyelashes, dewy glances and affected lisp. I should have killed him that night.