'Philip eventually examined the amulet for himself,' Thomas whispered. 'He described it to me. It was a sealed box, the size of a man's hand, slim and flat and smooth to the touch. It was pale, cream-coloured, but with coloured markings on its upper surface. It was made of neither wood nor ceramic nor metal – something none could identify. The Christians, huddled in their Mongol tent, exploring this thing, found they could not cut it with a knife, nor would fire bum it.'

But Bohemond himself discovered that if he pressed a certain marking on the upper lid, a green arrow, the box would speak to him – in good if stilted Latin, in a tiny insect's voice. The legates were startled, terrified, intrigued. After much praying, and the application of much of their precious stock of holy water, they gathered around the box to hear what the imp in it had to say.

The imp spoke clearly of the future: of the next day, and of the years to come.

Of the next day, it described in detail Ogodai's movements: the hour he would rise, the breakfast he would take, the councillors, ambassadors and generals he would meet, the letters he would dictate and have read to him, the wife he would lie with – and the cup of mare's milk laced with Chinese rice wine that he liked to drink in the middle of the day.

On that cup of mare's milk and wine, said the imp in its tiny voice, the future of the world depended. For it spoke then of what would happen if Ogodai lived – what would become of the world at the feet of the Mongols, in the future.

Mongol armies always advanced in the depths of winter, with their horses fat on the grasses of summer. So it would be, in just a few weeks, that they would fall at last on Vienna. The city would be plundered, torched and razed, and the Viennese scattered to starve on the plains. As the Mongols advanced further west, the Christian princes, hastily uniting, would raise another army, which would meet the Mongols in a pitched battle before Munich. The numbers were well matched. But the Christians would be lured by a false retreat into an ambush, a classic Mongol tactic. Munich would be smashed. The Mongols' advance would be barely interrupted.

Next the Mongol force would split once more into three detachments. The first would strike at the Low Countries, plundering the rich young trading cities there before shattering them and slaughtering the population in the usual way. Holland's dykes would be broken up; the sea would complete the Mongols' victory.

A second detachment would spend the summer grazing their horses in the plains around the smoking ruins of Paris, while students from what had been the finest university north of the Pyrenees scratched in the rubble for food.

The final Mongol detachment would meanwhile cross the Alps and descend on Italy. The vibrant modern cities of Milan, Genoa, Venice – all put to the sword, all destined never to rise again. And then there was the Eternal City. When the Mongols were done, it would be said that Rome had been reduced to the villages on the seven hills from which the great old city had once coalesced. The Mongols considered the Pope a prince, and therefore no hand would be raised against him. Instead the successor to Saint Peter would be thrust into a sack and trampled to death by horses.

The next season, the squabbling Christian kings of Spain would provide no serious resistance to the Mongol force that marched south through the Pyrenees. And then there was England. The Mongols had learned how to build boats in their campaigns in the far east. By the autumn of that year, London would burn.

So the conquest would be completed. With its great cities shattered, its monasteries and churches broken, Europe would be reduced to a shrunken population living in utter poverty in villages too small to be worth the plunder, ruled brutally by the khans' governors and tax-collectors.

Eventually, the imp said, the Mongols would withdraw, their empire withering away. But the damage would be permanent, Europe cut off from its own antiquity. And, worse, Christ would be lost from the world. With their priests slaughtered, the mass of the population slowly reverted to paganism, finding comfort in the gods they rediscovered in the trees and fields and rivers around them.

Bohemond, Philip and their companions listened to this dreadful account with growing horror.

But it need not be so, the imp whispered. Already grievous damage had been done to the cities of Russia, and even to the great Islamic civilisations of the east, which would never recover their sparkling brilliance of the past. But in the west, Christendom might yet be saved.

A tiny lid opened in the flat top of the box. Inside, revealed to the astonished men, was a pinch of crystals. This, the imp whispered, was a salt of quicksilver. If these crystals were dropped into Ogodai's milk and wine the next day the ruin of Christendom would be averted.

And then the box fell silent, and would not speak again, no matter what markings they pressed. The crystals sat in their little tray, silent, beckoning.

The Christians debated what this all meant. The soldiers like Philip discussed the Mongols' campaigns. The priests and monks explored the theological nature of the imp in the box: was it sent by God, or the devil?

And while they debated, Bohemond slipped away.

'By the end of the next day,' Thomas said, 'the Great Khan was doubled up in pain. His vomit was copious, while bloody diarrhoea hosed from his leathery backside. His doctors could do nothing. By the following morning he was unable even to pass water, and howled in agony. And by the end of the day after that he was dead. It was a horrible death – but not as dreadful as that inflicted on Brother Bohemond, who was discovered skulking in the Khan's tent.'

Like many of the other embassies, the Christian party packed up and fled in haste from the decapitated court. The Mongols' own messengers spread the news of the Khan's death to the generals and governors across their scattered domains.

'And that is why,' Thomas said, 'early in the year 1242, rather than press his conquest west, Sabotai turned back from the walls of Vienna. For all their conquests the Mongols remain tribesmen, bound by oaths of loyalty to their Khan. So when Ogodai died, their leaders were forced by their own laws to return in person to their homeland, to elect a new ruler.'

'And will they not return to Europe?' Saladin asked.

'They haven't yet. They have the rest of the world to occupy them. And as for the amulet – after the envoys had fled from the Mongol city, Philip told me they finally shattered its casing with rocks. Inside they found not the shrivelled corpse of an imp, but bits of wire. Metal discs, like coins, but blank. Other strange little sculptures.'

'Charms, perhaps,' said Saladin.

'Philip thought they were like bits of an engine. But what its function could be, how it worked – even what drove it, for there was no spring, no lever – he had no idea.'

Joan said, 'But whatever it was, why was this amulet put into the luggage of this boy Bohemond?'

'I think that's clear enough. It was put there so Bohemond should kill Ogodai. If he had lived, Christendom was lost. If he died, Christendom was saved. As simple as that. So he had to die.'

'But who could know this?… Ah,'Joan said. 'A prophet. Or-'

'Or a meddler with time,'Thomas said. 'A Weaver. A man, or an angel or a demon, with the power to speak to the past. A man stranded in this dismal future wrecked by the Khans, who managed to send back this imp-in-a-box – just as somebody, somewhere, somewhen, sent back – perhaps, perhaps! – the designs of your war machines to a young boy's addled head, and somebody else sent al-Hafredi back to the time of Charles Martel, and somebody else whispered in the ear of your ancestress Eadgyth, and, and…'