Ambrosias turned to Ulf and Wuffa. From the moment they had met he had seemed far more interested in the two young men than in the bishop or the girl, although there was no trace of Ammanius's lasciviousness in him. Ambrosias asked where the two of them were from, and they tried to explain, though their lack of a common geography was a problem: to Ambrosias they were both simply barbarians from beyond the old empire.

'And now you are here,' Ambrosias said, 'on the west coast of Britain, so far from home.'

'My people came to Britain,' Wuffa said, 'because of the sea. So my father told me. Every year the tides came higher. The beaches and cliffs eroded away. We were forced to retreat from our farms, which became waterlogged. But there was nowhere for us to go, for the land was full.'

'And so you came across the ocean. The sea rises, and we petty humans must flee. Before such forces, the coming and going of empires seems trivial – don't you think? But there may be deeper patterns yet.' Ambrosias leaned close to the two young men, peering into their faces. 'I once met an old man, a poor Briton fleeing west from the Angles, who told me of an ancient legend – it must date back thousands of years if it is true at all – that once you could walk across the ocean, or rather the floor of what is now the ocean. But the sea rose up. Sometimes, if you dig in the exposed sands on the coast you will find reindeer bones, even a stone tool or two. Do you think that we are all one, we people of the lands surrounding the ocean, that in a sense you are not migrants, you have simply come home?'

The idea was astounding to Wuffa. 'But how you could ever tell if that was true?'

Ammanius grunted grudging approval. 'An intellectual answer. I could make a scholar of you, wolf-boy, given time.'

Ulf, always more earthy than Wuffa, was uninterested. 'We have no legends of drowned lands. My people are warriors.'

'Ah, warriors,' said Ambrosias. 'The world is never short of warriors! When I was an infant my father presented me to the greatest warrior of all. Have you young blades ever heard of Artorius?'

They had not. Ambrosias seemed shocked.

Ammanius told them that as the German immigrants expanded from their coastal footholds and conflict spread across the island, the British found a general in Artorius, who had the authority to work across the boundaries of the province-states and organise significant resistance. He won a string of victories. 'Artorius' may have been a nickname, meaning 'the Bear man', perhaps a reference to his size. He was said to be the nephew of one of the last Roman commanders to have stayed at his post in Britain.

'All this was a century after the Roman severance,' Ammanius said. 'Artorius won peace for a generation. But all he really secured for his people was time.'

Wuffa asked, 'So why would this Artorius come here?'

'He retired here after a last battle,' Ambrosias said. 'Already an old man he was gravely wounded – worn down by the treachery and cowardice of his own men as much as the enemy's efforts. He died, here at Banna – on the Wall, the greatest monument of the empire to whose memory he devoted his life.' He was misty-eyed now. 'In another age they would have built him an arch here to rival any in Rome! And I, a child, was presented to him. He ruffled my hair! Here.' He knelt stiffly, presenting his bowed head to Wuffa. 'Touch my scalp. Go on!'

Wuffa glanced at the bishop, who shrugged. Wuffa laid his hand on the old man's head. His skin felt paper-thin, stretched over a fragile skull.

'Always remember. Tell your children!…'

After more conversation of this sort Ammanius stood and stretched. 'You've worn me out, sir, with your kind hospitality,' he said in his dry way.

Sulpicia stood. She wasn't about to be left alone with Wuffa and Ulf, even with the old man as chaperone. 'I will bid you goodnight too.' And, impulsively, she planted a light kiss on the crown of the old man's head.

Ambrosias smiled, pleased.

Wuffa and Ulf began to clamber to their feet too. But Ambrosias raised his hand in an unmistakable gesture. Wait. Let them go.

Ambrosias closed the door behind the bishop and the girl. Then, padding quietly, he went to a cupboard. 'I thought the bloody-nosed old fool would never tire. Our business is nothing to do with bishops, or even with that rather charming girl you both lust after.' Wuffa avoided Ulf's eyes. Ambrosias drew a scroll from the overfull cupboard. He glanced at the two of them, with a complicated mixture of regret and longing. 'Chance has brought you two here, in the wake of the bishop. But this was meant to be, the ancient words have been fulfilled.'

Wuffa glanced warily at Ulf. He felt his heart hammer; suddenly, in the presence of this limp old man who brandished nothing but a scroll of parchment, he felt fearful. He asked, 'The words of what?'

'This.' Ambrosias unrolled the parchment, holding it in trembling hands. 'It is the prophecy of Isolde.'

The document was yellowed with age, grimy with much handling. Wuffa recognised handwriting in somewhat ragged lines, perhaps scrawled in a hurry. But he couldn't read it. He couldn't even read his own name.

'So this is the prophecy,' prompted Ulf.

'Yes! It was written down at Isolde's birthing bed. For two hundred years my family have preserved it – two hundred years of waiting, reduced to this moment. I knew you would come. I knew.'

Ulf said cautiously, 'What do you mean? How did you know?'

'Because the light has returned to the sky.' Ambrosias pointed to the ceiling of his cramped room.

'The comet,' Wuffa breathed.

'Yes! And it is the comet around whose visits the prophecy is structured.' In a quavering voice Ambrosias began to read:

These the Great Years/of the Comet of God

Whose awe and beauty/in the roof of the world

Light step by step/the road to empire

An Aryan realm/THE GLORY OF CHRIST.

The Comet comes/in the month of June.

Each man of gold/spurns loyalty of silver.

In life a great king/in death a small man.

Nine hundred and fifty-one/the months of the first Year.

The Comet comes/in the month of September.

Number months thirty-five/of this Year of war.

See the Bear laid low/by the Wolf of the north.

Nine hundred and eighteen/the months of the second Year…

'And so on.' Ambrosias said reverently, 'This prophecy says that the comet will come again – and it has come before.'

'How can that be?' Ulf asked reasonably. 'Comets are like clouds. Aren't they? How can it come back?'

Ambrosias snorted. 'How could I possibly know? Ask Aristotle or Archimedes or Pythagoras – not me! All that matters is that it does so. And that is the basis of what the prophecy describes. My family, scholars all, refer to this as Isolde's Menologium, a calendar. For it is a calendar of a sort – but not of the seasons but of the comet's Great Years, each of them many of our earthly years long, marking out the events of man. Do you see?

'For example, the second stanza talks of the comet's appearance in the year of the Saxon revolt against the Vortigern. And then nine hundred and fifty-one months pass, marking the first Great Year, before the comet returns again, and then thirty-five months after that-'

'Nine hundred and fifty-one months,' Ulf mused. 'That's seventy years? Eighty?'

Ambrosias looked at him. 'You people are traders, aren't you? Illiterate or not, you can figure well enough.'

Wuffa said, 'You're going too fast. Why do you speak of the Vortigem?'

'Because that's what the prophecy says, in the first stanza. Look, here – ah, but you can't read it! "Each man of gold/spurns loyalty of silver. /In life a great king/in death a small man"…'