"He says he can help if we show him Chimalli's room," Yaotl said.

The hope that spread over Xoco's face was almost too painful to contemplate. "I don't guarantee anything," I said.

"It doesn't matter," Xoco said. "You're willing to help, and it's enough for me. Come."

Chimalli's room was wide, with only one reed mat, and clay toys strewn on the floor. It was clear only he slept inside it: again, an indication of wealth.

As soon as we entered, Chimalli walked straight to his mat and sat on it, his torso propped against the wall. Throughout, he never spoke a word. His gaze, from time to time, moved to me, and I had to look away. I had the feeling he saw through me, through everything I was, and judged me lacking.

Yaotl left soon after we entered the room: the pretext he gave was some manoeuvers with his regiment. The real reason, I feared, was that he did not want to be with Xoco. I wondered if they still loved each other, and thought it was one-sided.

I was left alone with Xoco, who had started fussing around her son.

"He's your only child?" I asked, moving about the room, not sure what I was looking for.

Her face twisted. "Yes," she said. "Yaotl's a good man. He says one child's enough to succeed him."

"You'd have wanted more?" I asked, and realised what a foolish question it was. For there, no doubt, was the root of their marriage failure.

"Had it been possible, yes," Xoco said. "It's no longer the case."

"I'm sorry," I said. I did not wait for her response, but instead started rummaging around the room.

The clay toys, models of warriors with their clubs and priests with their sacrificial knives, were amazingly detailed. Yaotl had spared nothing for his only son.

In the wicker chest by the mat were more toys: spinning tops of turquoise, and rag dolls. But still no trace of magic, sorcerous or otherwise. I had expected Chimalli to protest at seeing a stranger search through his possessions, but the boy remained sitting on his reed mat, staring at me in eerie silence.

To avoid looking back at him, I raised my eyes to the walls, gazing at the intricate frescoes a obviously painted by someone with talent. The painted gaze of Xochiquetzal stared back at me: the goddess of joy and flowers wore her quetzal feather headdress, and her intricate gold necklace. Her eyes had the same disturbing intensity as Chimalli's.

What had I missed? I thought again about what Yaotl had told me. One morning at dawn, Chimalli had refused to get up. The spell, then, had likely been cast during the previous night.

With all the slaves within the house, no one could have entered without raising an outcry. But that meant little. There were spells of invisibility. Yaotl's house was warded against intruders, but not against magic. A sorcerer could have found an easy access.

I combed the room again, but could find nothing. It unnerved me. Surely something that powerful would have left traces.

However... a curse like this, slowly spreading over Chimalli, would need to be renewed regularly. The spell-caster would have to come back, most likely every night.

Xoco was sitting by Chimalli's side when I rose from my second search.

"So?" she asked. Anxiety shone in her eyes.

"I found nothing," I said at last, ashamed to admit defeat. "I thought one of your husband's enemies might have been responsiblea" I left Xoco an opening, hoping to have more details, but she looked away.

"Please don't give up," she said.

I sighed. I could think of only one thing left to do. I asked Xoco, "Will you object if I stay here tonight? I have a feeling whatever comes for him does so at night."

It was a foolish idea. I still had no idea of whom I might face, or of his powers. But I had no other plan.

Xoco's gaze was uncertain. "I don't mind if you stay," she said at last. "Though I'll have to ask Yaotl."

As I suspected, Yaotl agreed to let me stay. He was far too worried about his child, and trying very hard not to show it, not to appear weak.

As the sun set, I made my preparations. I slaughtered a hummingbird, symbol of Huitzilpochtli, the god who protected the Mexica Empire, and drew wards in the bird's blood. The heart I put at the centre of Chimalli's room, and I said the words that would seal the protection.

What surprised me was that Xoco wanted to stay with me. I told her that for this to be effective, I needed to be alone with the boy. That I could not protect Chimalli, her and myself at the same time. Still she refused to leave. Finally, I won out.

When night fell, I was alone in the room. Chimalli was not sleeping; he was sitting on the reed mat, propped up against the wall. His eyes were trained on me. After a while, it became unbearable. I turned my gaze away from him, staring only at the tiled floor.

For what seemed like an eternity I remained seated beside my wards, silently repeating the spells of protection, suspecting they would not be enough.

It grew darker, and colder. The stars rose in the sky: I saw them beyond the door, but they lit nothing. Still I did not move. I had endured worse during my novitiate.

When a shadow fell across the door, I was wide awake.