Robert came home, seemingly full of disgust at what he had experienced of Moorish Spain. He became a warrior of God, eagerly taking the Cross when the Pope called the First Crusade.

Thomas said, ‘But he never forgot his strange experiences, the tale of the magical engines, his father’s prophecy, the future visions of al-Hafredi. In the end, driven by some sense of guilt perhaps - he may have felt it a betrayal of his father to just abandon it all - he told his own eldest son the whole story. And that son, mercifully for history and your family’s fortune, was more bookish than his father, and wrote it all down for us.

‘Now, by chance, not all of the Codex itself was lost. In the final struggle a scrap of it was torn away and ended up in Robert’s possession. It bore strange words . . .’ Thomas rummaged through scrolls on a low table before him for his copy of the fragment. ‘Ah, here we are.’ He ran his finger along a line of text.

BMQVK XESEF EBZKM BMHSM BGNSD DYEED OSMEM HPTVZ HESZS ZHVH

‘There were letters nearby too, preserved on the fragment, but ripped through. A D, perhaps, a V and an M - nothing else could be made out.’

Saladin read this over. ‘It’s in no language I ever saw.’

‘Reading isn’t your strong suit anyhow, son,’ Joan said, mocking him.

‘This is no known language,’ Thomas confirmed. ‘I believe this is a cipher - a code, perhaps of the type Caesar once used. There may be some key, which is lost. At any rate it was preserved, thanks to the transcription of the bookish son.’

‘Now,’ said Joan. ‘Here’s the most important thing for us, Saladin. Another of the three prophecies, the Testament of al-Hafredi, also fell by chance into Robert’s hands.’

‘Written on a bit of human skin,’ Thomas said with a certain morbid relish.

‘And this al-Hafredi has become our family’s own oracle.’

‘An oracle?’

‘I mean that literally, Saladin. One of Robert’s grandsons gave the material to Brother Thomas’s house to study and interpret it for us, and so they have, in the centuries since.’

Thomas said, ‘Al-Hafredi told of events to come - very broad-brush, but reliable none the less. And in particular he spoke of the advance of the Mongols. This followed the Islamic conquest of Europe, and he described it step by step.’

Saladin was trying to work this out, his face twisting. ‘The Muslims have never conquered Europe.’

‘True, but we can believe that the Mongols’ advance would have occurred as al-Hafredi described it, whether Islam conquered or not.’

Now Saladin seemed utterly baffled. ‘And Robert lived and died long before anybody had heard of the Mongols!’

‘He did indeed,’ Joan said. ‘It wouldn’t be much of a prophecy if it was the other way around, would it?’