‘It’s your fault!’ he had screamed, twisting and pulling away from Ahmad, who stood with his head bowed, absorbing the boy’s words as if they were punches.

‘It’s your fault,’ Altaïr had spat again, then sat on the brittle grass, burying his head in his hands, wanting to shut out the world. A few steps away, Ahmad, exhausted and beaten, had folded to the ground also.

Outside the citadel walls, the Saracens departed, leaving the headless body of Altaïr’s father behind for the Assassins to retrieve. Leaving wounds that would never heal.

For the time being Altaïr had stayed in the quarters he had shared with his father, with their walls of grey stone, rushes on the floor, a simple desk between two pallets, one larger, one smaller. He’d moved beds: he had slept in the larger one, so that he could smell his father’s smell, and he had imagined him sometimes, in the room, sitting reading at the desk, scratching away at a roll of parchment, or returning late at night to chide Altaïr for still being awake, then snuffing out his candle before retiring. Imaginings were all he had now, the orphan Altaïr. Those and his memories. Al Mualim had said he would be called in due course, when arrangements had been made for his future. In the meantime, the Master had said, if Altaïr needed anything, he should come to him as his mentor.

Ahmad, meanwhile, had been suffering from a fever. Some nights his ravings were heard throughout the citadel. Occasionally he screamed as if in pain, at other times like a man deranged. One night he was shouting one word over and over again. Altaïr had pulled himself from his bed and gone to his window, thinking that what he heard was his father’s name.

It was. ‘Umar.’ Hearing it was like being slapped.

‘Umar.’ The shriek seemed to echo in the empty courtyard below. ‘Umar.’

No, not empty. Peering more closely, Altaïr could make out the figure of a child of about his age, who stood like a sentinel in the soft early-morning mist that rippled across the training yard. It was Abbas. Altaïr barely knew him, just that he was Abbas Sofian, the son of Ahmad Sofian. The boy had stood listening to his father’s demented ravings, perhaps offering silent prayers for him, and Altaïr had watched him for a few heartbeats, finding something to admire in his silent vigil. Then he had let his curtain drop and returned to his bed, putting his hands over his ears so that he could no longer hear Ahmad calling his father’s name. He had tried to breathe in his father’s scent and realized that it was fading.

They said that Ahmad’s fever had abated the next day, and that he had returned to his quarters, albeit a broken man. Altaïr had heard that he lay on his bed attended to by Abbas. That he had lain that way for two days.

The next night Altaïr was awoken by a sound in his room and lay blinking, hearing somebody moving about, feet that went to the desk. A candle was put down that threw shadows on the stone wall. It was his father, he thought, still half asleep. His father had come back for him, and he sat up, smiling, ready to welcome him home and be chided by him for being awake. At last he had woken from a terrible dream in which his father had died and left him alone.

But the man in his room was not his father. It was Ahmad.

Ahmad was standing at the door, emaciated within his white robe, his face a pale mask. He wore a faraway, almost peaceful expression, and he smiled a little as Altaïr sat up, as though he didn’t want to frighten the boy. His eyes, though, were sunken dark hollows as if pain had burned the life from within him. And in his hand he held a dagger.

‘I’m sorry,’ he said, and they were the only words he spoke, his last words, because next he drew the knife across his throat, opening a gaping red mouth in his own neck.

Blood swept down his robe; bubbles of it formed at the wound on his neck. The dagger dropped with a clunk to the floor and he smiled as he slid to his knees, his gaze fixed on Altaïr, who sat rigid with fear, unable to take his eyes from Ahmad as the blood poured from him, draining out of him. Now the dying man lolled back on his heels, at last breaking that ghastly stare as his head dropped to the side, but he was prevented from falling backwards by the door. And for some heartbeats that was how he remained, a penitent man, kneeling. Then at last he fell forward.

Altaïr had no idea how long he sat there, weeping softly and listening to Ahmad’s blood spreading thickly across the stone. At last he found the courage to step out of bed, taking the candle and carefully skirting the bleeding horror that lay on the floor. He pulled his door open, whimpering as it made contact with Ahmad’s foot. Outside the room at last, he ran. The candle snuffed out but he didn’t care. He ran until he reached Al Mualim.

‘You must never tell anyone of this,’ Al Mualim had said, the next day. Altaïr had been given a warm spiced drink, then spent the rest of the night in the Master’s chambers, where he had slept soundly. The Master himself had been elsewhere, attending to Ahmad’s body. So it had proved the next day, when Al Mualim returned to him, taking a seat by his bed.

‘We shall tell the Order that Ahmad left under cover of darkness,’ he said. ‘They may draw their own conclusions. We cannot allow Abbas to be tainted with the shame of his father’s suicide. What Ahmad has done is dishonourable. His disgrace would spread to his kin.’

‘But what of Abbas, Master?’ said Altaïr. ‘Will he be told the truth?’

‘No, my child.’

‘But he should at least know that his father is –’

‘No, my child,’ repeated Al Mualim, his voice rising. ‘Abbas will be told by no one, including you. Tomorrow I shall announce that you are both to become novices in the Order, that you are to be brothers in all but blood. You will share quarters. You will train and study and dine together. As brothers. You will look after each other. See no harm comes to the other, either physical or by other means. Do I make myself clear?’

‘Yes, Master.’

Later that day Altaïr was installed in quarters with Abbas. A meagre room: two pallets, rush matting, a small desk. Neither boy liked it but Abbas said he would be leaving shortly, when his father returned. At night he was fitful and sometimes called out in his sleep, while in the next bed Altaïr lay awake, afraid to sleep in case the nightmares of Ahmad uncoiled themselves and came to him.

They did. Ahmad had come to him at night ever since. He came with a dagger that gleamed in the dancing candlelight. Slowly he drew the blade across his own throat, grinning as he did so.