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 would slowly revert 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.