It was the faded trail of a beast of shadows, eager to feast on a human heart, to receive its promised reward.

"Two magics," I said aloud. I could have wept. Why hadn't I seen that before?

Teomitl had followed me into the courtyard; he stood, silently watching the pine tree at the centre as if he could extract some meaning from its twisted shadow. "Two spells?" he asked.

"I didn't think..." I attempted to make sense of what I'd seen. "Someone summoned a beast of shadows from Mictlan. And someone else a someone in this calmecac a added nahual magic on top of it, to cover the trail."

"I don't see the pointa" Teomitl started.

"Beasts of shadows aren't common," I said. "You can track them." I could track it. I could find Eleuia. But a full day and night had elapsed since her disappearance. The beast, if it had not killed her, had had time to do whatever its summoner had wished it to.

"You couldn't track a nahual?" Teomitl asked, with faint contempt.

I shook my head. "Too many of them. And the magic dissipates in daylight. But using one magic to cover another..." That had been a masterful stroke; an uncommon idea that required a great knowledge of magic.

Who had captured Eleuia, and why? Was it Mahuizoh? I didn't know. But I didn't think he'd have the skill to cover his tracks, even if he had breached the boundary between the underworld and the Fifth World.

Anyone with the proper knowledge could summon a beast of shadows. But if I could find the beast, I would learn who its summoner was: a beast of shadows was imprinted with the few moments that had followed its entrance into the Fifth World. It would remember its summoner.

"I see." Teomitl's face was set. "Now what?"

"Now you go home," I said.

Teomitl shook his head. "No."

It was late; I was tired, and not in a mood to negotiate. "You don't understand," I said. "It's going to get dangerous. Very dangerous."

Impatience was etched into every feature of his face. "All the more reason for you not to go into this alone."

"I've been tracking beasts of shadows for ten years," I said.

"Yes," Teomitl said. "But you're tired."

I started. "How do you know?"

He shrugged. "I can read it. It's not so hard, Acatl-tzin."

Not only was I tired, it showed even to callow youths. "I don't need help," I said.

"But you might," Teomitl said.

"Looka" I started, and stifled the yawn that threatened to distort my face.

"I'll be careful," Teomitl said. "I know how to fight." "It's not a warrior's fight."

"No," Teomitl said. "But it is still a fight."

"And you're that eager to get into trouble?"

"To prove myself." The hunger in his gaze was palpable: an obsession that was eating him from inside.

"Haven't you proved yourself already?" I asked. "You took a prisoner."

He snorted. "With my comrades' help. That's no feat of arms."

I sighed, and presented what I hoped would be a decisive argument. "Understand this," I said. "If you do help me, you can't breathe a word about it."

Most youths would have refused at that point. For what is the use of feats, if you cannot boast of them to your comrades? But Teomitl tossed his head, contemptuously. "I don't care about my peers' opinions. Is it a 'yes', then?"

I had exhausted my arguments; and time was running short. "You'll do as I say," I snapped.

Teomitl smiled widely. "Of course."

"And don't put yourself in danger needlessly. I don't need a death on my conscience." But he would not go the way of my apprentice, Payaxin, wouldn't die because of a mistake.

Teomitl shook his head, as if implying that needless deaths were utter foolishness.

"Let's go," I said, aware I'd just been played on, with the same skill as a musician on the flute. For all his arrogance, Teomitl was a shrewd judge of men. Too shrewd for his own good, perhaps.

Before Teomitl and I started tracking down the beast of shadows, I did take the precaution of sending someone to the Imperial Palace, to see if there had been any further developments in Neutemoc's case.

The offering priest a Palli, the burly nobleman's son who usually guarded the storehouse a returned as Teomitl and I were in the armoury of the temple, lifting throwing spears and arrows to find those tipped with magical obsidian.

Nothing further had happened. Everyone, High Priest Acamapichtli included, had gone to sleep. Lucky men. Fatigue was making my head light, and I had some trouble focusing on the objects Teomitl and I had spread on the ground: three spears with shell-grips, a batch of arrows still in their quiver, and two macuahitl swords, wooden clubs studded with razor-sharp obsidian shards. Everything shone, faintly, with the hues of underworld magic: a sickly light that seemed to diminish all it touched.

"I have a sword," Teomitl said, faintly annoyed. "A good one." His sword was well-made, but it would not be enough against the supernatural.

"You'll need one of those," I said, pointing to the swords on the ground. "To fight the beast. What do you know about beasts of shadow?"

A quick, fluid shrug. "Not much. They live on the eighth level of Mictlan. You need a jade bead to placate them."

I nodded. "They're made of shadows, of the darkness that lay over the world in the very first days. Jade will slow them down but not stop them. And they feast on human hearts."

Teomitl nodded. "Many things do."

"Indeed." The gods, the Celestial Women. Nothing was as precious as blood; and the most precious thing of all was the heart, which gathered all the blood and distributed it around the body.

"The beasts hate light," I said, curtly, picking up one of the sturdiest swords and hefting it. A faint touch of Mictlan's magic spread to my arm, stilling the blood in my veins, and numbing my skin. "Starlight won't bother them much, but moonlight or sunlight will weaken them. Sunrise might even destroy them."

"How do you kill them?" Teomitl asked.

"This sword has magical obsidian: Lord Death's gift to us. Stab their chest, and they'll die like any animal." I handed it to him. What I didn't say was that they were fierce fighters, more than a match for both of us.

"Hmm," Teomitl said. He was tense again, impatient to move on.

We repaired to one of the furthest rooms, after warning Ichtaca not to disturb us. The room was a simple, subdued affair, its only furniture a handful of wicker chests, its walls blank save for a spider-and-owl frieze running at head height. A small, discreet limestone altar was at the back, Mictlantecuhtli's skull symbol carved in its centre.

I knelt on the floor, feeling the coldness of the beaten earth through my bare knees. Before me, a wicker cage held a rabbit and a barn owl: a small bird with the sharp eyes of a hunter, blinking in the torchlight. On my left was a jade replica of a human heart, in exquisite detail. On my right was a jade plate, representing the journey of the soul through Mictlan, from the crossing of the River of Souls on the first level, to the ninth and final level, the Throne of Mictlantecuhtli, God of the Dead.

I slit the throat of the rabbit with one of my obsidian knives, and carefully drew a quincunx on the floor: the four-armed cross with its fifth central point.

"Whatever happens, don't move," I said to Teomitl. He stood outside that quincunx, outside the underworld's zone of influence. "And don't step over the line."

He shrugged arrogantly. Who are you taking me for? he seemed to say.

Already, I regretted my decision to bring him with me. But I wasn't about to go back on my word.

"In darkness they dwell In darkness they feast They eat, they consume their preys..."

I reached for the owl and swiftly opened up its chest in a shower of blood. I retrieved its heart, and laid it on the jade plate: between the seventh and eighth level of the underworld, over a crude drawing of a shadowy beast. Blood seeped onto the plate, spread outwards like a scarlet flower opening its petals.

"In darkness they feast They eat, they consume their preys All save one..."

Shadows gathered around the room, pooled to fill the quincunx, until I knelt in absolute darkness. A wet, heavy breath blew down my neck: the breath of a huge animal, waiting for its food.

I didn't falter. I knew all too well the cost of failure. Death, if I was lucky; utter oblivion if I wasn't.

"All save one One is lost One runs under the light Under the light of stars and moons One is lost..."

I laid the tip of my obsidian knife against the jade heart, so that the remains of the owl's blood seeped into the stone. The jade grew darker, but it pulsed now, pulsed like a living heart.

"A jade heart to find the eater of hearts Who feasts on the living Under the light of stars and moons He comes to Mictlan, the Place of Fear, the Place of Death A jade heart to find the eater of hearts."

The jade heart went completely black. I reached out to enfold it in my hands, ignoring the searing pain that spread from the stone into my skin, and felt it beat under my fingers: a slow, regular rhythm that started in the rightmost ventricle and moved upwards, into my arm.

Slowly, carefully, I rose, keeping the heart in the same position. The beat didn't falter.

The shadows were gradually dispelled by torchlight; I could see Teomitl's shocked face, trying to reassemble itself into its usual haughty mask a and the body of the owl, blood pooling under it, the red flower blossoming on the jade plate.

Still standing within my quincunx, I turned to face each direction in turn. When I turned north, east or west, the beat of the jade heart went completely still. But when I faced south, towards the Itzapalapan causeway, the heart sprang to life under my fingers.

It had worked, then; and the beast we sought was somewhere in that direction.

Teomitl insisted on bringing a purse filled with medicinal herbs, as well as his new sword. He kept rubbing the weapon, as if its touch were subtly wrong. I couldn't blame him: the obsidian shards embedded in the wood were charged with enough magic to send anyone into oblivion. The veil hanging around him seemed to be weaker around the sword, as if the magics were fighting one another, but it did not appear to be serious a for which I was eternally grateful.

As we exited the temple and headed southwest towards the district of Moyotlan, I asked, "You do know about the magic?"

He looked puzzled. "Which magic?"

"Around you?"

The way Teomitl attempted to look at his arms and legs convinced me he hadn't known. But he didn't look happy about it: his face darkened, a change of mood that was visible even in the wavering light of his torch.

"A protective spell some fool laid on me without my consent," he muttered, darkly. "Nothing worth worrying about."

I did wonder, though: it was powerful magic, and I wasn't entirely sure how it would withstand the assaults of a beast of shadows a some spells just shattered, crippling the people on whom they'd been laid. "You'd better stay back," I said.

Teomitl shook his head and didn't answer.

Under my fingers, the heart beat at its steady rhythm as we ran through the deserted streets. The grand houses became smaller, turning from adobe to mud, the flat roofs replaced by high, tapered ones painted with abstract patterns; finally turning into the humble mud dwellings of peasants, ringed by fields of maize. We were almost at the lake shore.

Teomitl's hands held the torch steady as we ran out of the city altogether, and found ourselves in the midst of dried maize husks, crunching under our feet.

I turned the heart right and left; and it beat again, in the direction of a group of small, squat islands on the edge of the lake.

Teomitl saw the way I faced, and groaned. "Oh no. Not the Floating Gardens."