‘Die in pain, mortal!’ Before I could raise my sword in defence, another daemon’s claws raked my shoulder. If I’d been wearing armour, no problem, but I was still in my school uniform. The thing’s talons sliced my shirt open and tore into my skin. My whole left side seemed to explode in pain.

Nico kicked the monster away and stabbed it. All I could do was collapse and curl into a ball, trying to endure the horrible burning.

The sound of battle died. Thalia and Nico rushed to my side.

‘Hold still, Percy,’ Thalia said. ‘You’ll be fine.’ But the quiver in her voice told me the wound was bad. Nico touched it and I yelled in pain.

‘Nectar,’ he said. ‘I’m pouring nectar on it.’

He uncorked a bottle of the godly drink and trickled it across my shoulder. This was dangerous – just a sip of the stuff is all most demigods can stand – but immediately the pain eased. Together, Nico and Thalia dressed the wound and I only passed out a few times.

I couldn’t judge how much time went by, but the next thing I remember I was propped up with my back against a rock. My shoulder was bandaged. Thalia was feeding me tiny squares of chocolate-flavored ambrosia.

‘The Keres?’ I muttered.

‘Gone for now,’ she said. ‘You had me worried for a second, Percy, but I think you’ll make it.’

Nico crouched next to us. He was holding the potted carnation. Only five petals still clung to the flower.

‘The Keres will be back,’ he warned. He looked at my shoulder with concern. ‘That wound… the Keres are spirits of disease and pestilence as well as violence. We can slow down the infection, but eventually you’ll need serious healing. I mean a god’s power. Otherwise…’

He didn’t finish the thought.

‘I’ll be fine.’ I tried to sit up and immediately felt nauseous.

‘Slow,’ Thalia said. ‘You need rest before you can move.’

‘There’s no time.’ I looked at the carnation. ‘One of the daemons mentioned Iapetus. Am I remembering right? That’s a Titan?’

Thalia nodded uneasily. ‘The brother of Kronos, father of Atlas. He was known as the Titan of the West. His name means ‘the Piercer’ because that’s what he likes to do to his enemies. He was cast into Tartarus along with his brothers. He’s supposed to be still down there.’

‘But if the sword of Hades can unlock death?’ I asked.

‘Then maybe,’ Nico said, ‘it can also summon the damned out of Tartarus. We can’t let them try.’

‘We still don’t know who them is,’ Thalia said.

‘The half-blood working for Kronos,’ I said. ‘Probably Ethan Nakamura. And he’s starting to recruit some of Hades’s minions to his side – like the Keres. The daemons think that if Kronos wins the war, they’ll get more chaos and evil out of the deal.’

‘They’re probably right,’ Nico said. ‘My father tries to keep a balance. He reins in the more violent spirits. If Kronos appoints one of his brothers to be the lord of the Underworld –’

‘Like this Iapetus dude,’ I said.

‘– then the Underworld will get a lot worse,’ Nico said. ‘The Keres would like that. So would Melinoe.’

‘You still haven’t told us who Melinoe is.’

Nico chewed his lip. ‘She’s the goddess of ghosts – one of my father’s servants. She oversees the restless dead that walk the earth. Every night she rises from the Underworld to terrify mortals.’

‘She has her own path into the upper world?’

Nico nodded. ‘I doubt it would be blocked. Normally, no one would even think about trespassing in her cave. But if this demigod thief is brave enough to make a deal with her –’

‘He could get back to the world,’ Thalia supplied. ‘And take the sword to Kronos.’

‘Who would use it to raise his brothers from Tartarus,’ I guessed. ‘And we’d be in big trouble.’

I struggled to my feet. A wave of nausea almost made me black out, but Thalia grabbed me.

‘Percy,’ she said, ‘you’re in no condition –’

‘I have to be.’ I watched as another petal withered and fell off the carnation. Four left before doomsday. ‘Give me the potted plant. We have to find the cave of Melinoe.’

As we walked, I tried to think about positive things: my favourite basketball players, my last conversation with Annabeth, what my mom would make for Christmas dinner – anything but the pain. Still, it felt like a sabre-toothed tiger was chewing on my shoulder. I wasn’t going to be much good in a fight, and I cursed myself for letting down my guard. I should never have got hurt. Now Thalia and Nico would have to haul my useless butt through the rest of the mission.

I was so busy feeling sorry for myself I didn’t notice the sound of roaring water until Nico said, ‘Uh-oh.’

About fifteen metres ahead of us, a dark river churned through a gorge of volcanic rock. I’d seen the Styx, and this didn’t look like the same river. It was narrow and fast. The water was black as ink. Even the foam churned black. The far bank was only ten metres across, but that was too far to jump, and there was no bridge.

‘The River Lethe.’ Nico cursed in Ancient Greek. ‘We’ll never make it across.’

The flower was pointing to the other side – towards a gloomy mountain and a path leading up to a cave. Beyond the mountain, the walls of the Underworld loomed like a dark granite sky. I hadn’t considered that the Underworld might have an outer rim, but this appeared to be it.

‘There’s got to be a way across,’ I said.

Thalia knelt next to the bank.

‘Careful!’ Nico said. ‘This is the River of Forgetfulness. If one drop of that water gets on you, you’ll start to forget who you are.’

Thalia backed up. ‘I know this place. Luke told me about it once. Souls come here if they choose to be reborn, so they totally forget their former lives.’

Nico nodded. ‘Swim in that water and your mind will be wiped clean. You’ll be like a newborn baby.’

Thalia studied the opposite bank. ‘I could shoot an arrow across, maybe anchor a line to one of those rocks.’

‘You want to trust your weight to a line that isn’t tied off?’ Nico asked.

Thalia frowned. ‘You’re right. Works in the movies, but… no. Could you summon some dead people to help us?’

‘I could, but they would only appear on my side of the river. Running water acts as a barrier against the dead. They can’t cross it.’

I winced. ‘What kind of stupid rule is that?’

‘Hey, I didn’t make it up.’ He studied my face. ‘You look terrible, Percy. You should sit down.’

‘I can’t. You need me for this.’

‘For what?’ Thalia asked. ‘You can barely stand.’

‘It’s water, isn’t it? I’ll have to control it. Maybe I can redirect the flow long enough to get us across.’

‘In your condition?’ Nico said. ‘No way. I’d feel safer with the arrow idea.’

I staggered to the edge of the river.

I didn’t know if I could do this. I was the child of Poseidon, so controlling salt water was no problem. Regular rivers… maybe, if the river spirits were feeling cooperative. Magical Underworld rivers? I had no idea.

‘Stand back,’ I said.

I concentrated on the current – the raging black water rushing past. I imagined it was part of my own body, that I could control the flow, make it respond to my will.

I wasn’t sure, but I thought the water churned and bubbled more violently, as if it could sense my presence. I knew I couldn’t stop the river altogether. The current would back up and flood the whole valley, exploding all over us as soon as I let it go. But there was another solution.

‘Here goes nothing,’ I muttered.

I raised my arms like I was lifting something over my head. My bad shoulder burned like lava, but I tried to ignore it.

The river rose. It surged out of its banks, flowing up and then down again in a great arc – a raging black rainbow of water six metres high. The riverbed in front of us turned to drying mud, a tunnel under the river just wide enough for two people to walk side by side.

Thalia and Nico stared at me in amazement.

‘Go,’ I said. ‘I can’t hold this for long.’

Yellow spots danced in front of my eyes. My wounded shoulder nearly screamed in pain. Thalia and Nico scrambled into the riverbed and made their way across the sticky mud.

Not a single drop. I can’t let a single drop of water touch them.

The River Lethe fought me. It didn’t want to be forced out of its banks. It wanted to crash down on my friends, wipe their minds clean and drown them. But I held the arc.

Thalia climbed the opposite bank and turned to help Nico.

‘Come on, Percy!’ she said. ‘Walk!’

My knees were shaking. My arms trembled. I took a step forward and almost fell. The water arc quivered.

‘I can’t make it,’ I called.

‘Yes you can!’ Thalia said. ‘We need you!’

Somehow, I managed to climb down into the riverbed. One step, then another. The water surged above me. My boots squished in the mud.

Halfway across, I stumbled. I heard Thalia scream, ‘No!’ And my concentration broke.

As the River Lethe crashed down on me, I had time for one last desperate thought: Dry.

I heard the roar and felt the crash of tons of water as the river fell back into its natural course. But…

I opened my eyes. I was surrounded by darkness, but I was completely dry. A layer of air covered me like a second skin, shielding me from the effects of the water. I struggled to my feet. Even this small effort to stay dry – something I’d done many times in normal water – was almost more than I could handle. I slogged forward through the black current, blind and doubled over with pain.

I climbed out of the River Lethe, surprising Thalia and Nico, who jumped back a good two metres. I staggered forward, collapsed in front of my friends, and passed out cold.

The taste of nectar brought me around. My shoulder felt better, but I had an uncomfortable buzz in my ears. My eyes felt hot, like I had a fever.

‘We can’t risk any more nectar,’ Thalia was saying. ‘He’ll burst into flames.’

‘Percy,’ Nico said. ‘Can you hear me?’

‘Flames,’ I murmured. ‘Got it.’

I sat up slowly. My shoulder was newly bandaged. It still hurt, but I was able to stand.

‘We’re close,’ Nico said. ‘Can you walk?’

The mountain loomed above us. A dusty trail snaked up a hundred metres or so to the mouth of a cave. The path was lined with human bones for that extra-cosy feel.

‘Ready,’ I said.

‘I don’t like this,’ Thalia murmured. She cradled the carnation, which was pointing towards the cave. The flower now had two petals left, like very sad bunny ears.

‘A creepy cave,’ I said. ‘The goddess of ghosts. What’s not to like?’