‘Eripe me . . . Acilius Glabrio sent an invitation to Postumus, pledging his support if the rebel crossed the Alps to overthrow the Emperor.’

‘Most likely the death of his cousin in Gaul prompted him to act.’ Murena had always been quick to pick things up. ‘He blames Gallienus.’

‘Quite so,’ Volusianus said. ‘But Postumus has made repeated public proclamations that he is content to rule what he holds. In his reply he had to adapt the lines of the Sibyl to Palinurus to restate that, for him, Gaul, Britain and Spain are empire enough. But in the final line, the one originally addressed to Aeneas – pull the blade from its scabbard – Postumus encourages Acilius Glabrio to assassinate Gallienus, and bid for the throne here in Rome himself.’

‘It is more than enough evidence to condemn Acilius Glabrio for treason,’ Murena said. ‘Ironically, he is at Cumae, consulting the oracle. There are frumentarii watching him. Shall I send word to make the arrest?’

‘Not yet. Keep him under close observation. Have all his correspondence intercepted, copied, resealed and then delivered. That it has been tampered with must not be detected by the recipients. It is a skill for which the frumentarii are notorious,’ Volusianus said. ‘If Acilius Glabrio has the courage to follow the advice of Postumus, he will first look for support. Most likely he will implicate other malcontents.’

‘But is there a danger he might act alone?’

‘If he is well watched, we can prevent him getting near enough to the Emperor to strike the blow.’ Volusianus thought for a moment. ‘Acilius Glabrio has many relatives in high places. If he is condemned, they must share his fate. The scandal might delay next year’s campaign.’

‘If the Emperor recovers, he intends to cross the Alps again?’

‘That is Gallienus’s intention. After all, Postumus did murder his son.’ A knowing look passed across the bluff features of Volusianus. ‘But other things might intervene.’

Murena felt a chill of apprehension that he prayed did not appear on his face.

‘Any news from Severus?’ Volusianus asked.

‘None,’ Murena said. ‘But none is to be expected. They are deep behind enemy lines.’

‘They should be near the Caspian by now. If they succeed, we should get word from the Black Sea by the end of winter.’

Murena said nothing.

Volusianus gazed at the pincers and the claws and the other instruments of torture, as if he were a shrewd farmer at market, wondering which to buy. ‘You have heard the rumours that Shapur has threatened to mutilate the Emperor?’

‘From a good source,’ Murena said, ‘one of our spies at Ctesiphon. The King of Kings announced that he might release Valerian from captivity, but that first he would cut off the Emperor’s nose. The Persian knows that in Rome no mutilated man can wear the purple.’

‘The gods willing, it will not come to that.’ Volusianus sighed, dismissing the unhappy train of thought.

‘What shall I do with the messenger?’