'And it was not Abd al-Rahman who was killed that day,' Sihtric said grimly, 'but Charles.' The Franks, demoralised by defeat and convulsed by a succession dispute, could offer the Moors little further resistance.

And the gate to the Great Land was open.

The subsequent Moorish expansion across Gaul and then Germany was like the story of their conquest of Spain – if anything more dramatic. Then there was England. The Umayyad caliphate had long been a great naval power in the Mediterranean; the ocean between England and Gaul offered them no resistance, and nor did the squabbling Saxon kings when Moorish ships sailed up the estuaries of the Thames and the Severn and the Tyne.

'By the year of Our Lord 793,' Sihtric said, 'in which your Viking ancestors first raided England, Orm, there were Moors in Paris and in Rome. Even Constantinople had fallen, after a decade-long siege from both east and west. After that the political history of Moorish Europe was no simpler or less fractious than that of al-Andalus, but overall the Moorish grip on the Great Land never loosened.

'There was to be no Jorvik, no Danelaw, and no Normandy. There was no battle at Hastings, no Norman invasion of England – for there were no Normans! The emirs never allowed Vikings to settle on their territory as the Frankish kings did.'

Armed with the legacy of antiquity, the Moors were able to make the northern lands flourish as they had al-Andalus. Populations rose steadily, and gained in wealth and health – and, just as steadily, converted to Islam. There was an intellectual revolution, and marvellous medicines and machines transformed the lives of the people.

'The greatest mosque in Europe was built in Seville, but the second grandest was in Paris,' Sihtric said. 'The greatest library in the world was in London. Think of that!

'And it was in a Moorish London that a young man called al-Hafredi was to be born. In a few words he sketches his London for us, a London where minarets and marble-columned palaces rise within the old Roman walls, and the cries of the imams drift across the Thames.

'Al-Hafredi claimed he had come from a far future, a thousand years beyond the Muslim conquest. And he sketches that millennium – a future that was already history to him. There will be invaders,' Sihtric said. 'From the east. A wave of savage horsemen, bursting out of Asia. The Muslim rulers, fractious as ever, will be unable to stand before them. Al-Hafredi details their progress. But the nomads' world empire will be brief, gone in a few generations, leaving only memories of distant lands.

'In the next age, plague. Many will fall. It would have been far worse, says al-Hafredi, if not for Muslim medicine.

'And in the age after that there will be a terrible war, a war of the Silk Road, as empires of east and west fall on each other. The war will engulf the whole world, and will last another century. And it will be won by machines. I imagine engines like mine, like Aethelmaer's, or even more destructive, born in the fecund minds of warriors and those who serve them.

'The war, long and bloody, will be won by the Muslims. In the end Islam will hold sway across all the world as it is known, from Scandinavia to Africa, from Ireland to India and the lands beyond. And ships bearing the crescent banner will sail far beyond the horizon in search of new lands to conquer, new peoples to convert.'

'And somewhere in this future Islamic world,' Orm said, 'your friend al-Hafredi broods, unforgiving.'

'Yes. And here is the strangest part of the story. Just as in al-Andalus, Christianity will be tolerated – even a thousand years after Abd al-Rahman. But the bitter monks of Lindisfarne and elsewhere will be pinpricks of Christianity in a Muslim map. Christ will live on through them, for al-Hafredi quotes Matthew, chapter eighteen: "For where two or three are gathered together in my name, there I am in the midst of them." But Islam will be everywhere else. The situation will be intolerable, the whole world lost – and in the end, al-Hafredi feared, Christianity would be extinguished altogether. Something must be done.

'So the devious monks will steal one of the Moors' own marvellous engines, and hatch a plot to use against its inventors. Don't ask me how it is done – I barely understand the what, let alone the how. But they will find a way to hurl one man across history, just as my crossbow will hurl a bolt across the sky, just as your Witness sent her words across the firmament to Eadgyth – they will hurl him, naked and alone, into another time and place.'

Orm saw it. 'They sent al-Hafredi from Lindisfarne, in this future century, to Poitiers, in the deep past.'

'That is what al-Hafredi tells us happened to him,' Sihtric said firmly. 'And, following the mission that had been devised for him, he made his way to Odo, and turned that weak man's mind around.'

Orm tried to take all this in. 'If that was his mission, he succeeded. This other Europe is now extinguished altogether. The mosque of Paris, the great library in London-'

'They never existed – and never will.'

Orm thought of the beauties he had seen here in al-Andalus, and he remembered the Normans' harrying of the English north. 'Do you think this world, our world, is a better one, Sihtric?'

Sihtric sniffed. 'That other wasn't a Christian world; it deserved to vanish.'

Orm studied the vellum again, and stroked it gingerly with a fingertip. 'What is this stuff – goat, lamb? Why didn't your long-dead scribe use a better quality bit of leather? These wounds are odd. This one looks like an arrow puncture. Was this animal hunted down?'

Sihtric eyed him. 'Can't you guess what this is, Viking? A pity; I thought you were showing imagination for once. Think about it. Al-Hafredi brought back an account of his own lost future in written form; perhaps he feared that his crossbow-shot across time would leave him dead, but that his message might do some good even so… And yet he travelled naked.'

Orm saw it. He drew his hand back. 'He bore his message on his body.'

Sihtric traced the letters on the bit of vellum with his finger. 'Tattooed across his back, compressed Bible quotations and all. This evidence of a stitched-up arrow wound is a detail that adds veracity to the whole saga, doesn't it? And when he died, stranded centuries out of his own time, the monks who tended him cut the skin off his back, and treated it as they would any bit of calfskin to be used for scribing.'

Orm stared at the bit of human skin, flayed off the body of a man from a vanished future. He felt obscurely angry. What strange world was he living in that such things could be possible? 'Tell me what you intend to do about all this.'

'I intend,' Sihtric said coldly, 'to follow al-Hafredi's example.'

'What do you mean?'

'I have seen an Islamic future, through al-Hafredi's words. The Moors may have been turned back at Poitiers, but Islam is still strong-rampant. I will not allow such a victory to come about.' He smiled coldly. 'Like al-Hafredi, I will use the Moors' own wealth and learning against them.'

Orm said slowly, 'So you intend to develop your engines with Moorish money. Then you will hand over the weapons to the Christian kings. And with those engines, all of Islam will be destroyed.'

'That's the plan. Simple, isn't it? It may be that I won't live to see the project completed, of course. But that's of no relevance. The march of Christendom transcends a mere human life.'

'But to betray your sponsor-'

'It won't be hard. You've met him. The vizier's no fool, but he is a drunk. He's not hard to manipulate.'

Orm wasn't so sure about that. 'And what of your conscience, Sihtric? What of the helpless millions whose destinies you plan to deflect? What of their souls? Does that not trouble you?'

'No, Orm, I am not troubled. There is another line from Revelation here. Chapter three: "I will not blot out his name out of the book of life." But al-Hafredi did, and I will. You came here with your head full of vague plans to oppose me, didn't you? Your wife's nonsense about the Dove. But you're full of doubt. Lesser men always are. But I, I am doing God's work – I am sure of that – that is all that matters.'