As if in response, a hissing sound echoed down the mountain. White mist billowed from the cave like someone had turned on a dry-ice machine.

In the fog, an image appeared – a tall woman with dishevelled blonde hair. She wore a pink bathrobe and had a wine glass in her hand. Her face was stern and disapproving. I could see right through her, so I knew she was a spirit of some kind, but her voice sounded real enough.

‘Now you come back,’ she growled. ‘Well, it’s too late!’

I looked at Nico and whispered, ‘Melinoe?’

Nico didn’t answer. He stood frozen, staring at the spirit.

Thalia lowered her bow. ‘Mother?’ Her eyes teared up. Suddenly she looked about seven years old.

The spirit threw down her wine glass. It shattered and dissolved into the fog. ‘That’s right, girl. Doomed to walk the earth, and it’s your fault! Where were you when I died? Why did you run away when I needed you?’

‘I – I –’

‘Thalia,’ I said. ‘It’s just a shade. It can’t hurt you.’

‘I’m more than that,’ the spirit growled. ‘And Thalia knows it.’

‘But – you abandoned me,’ Thalia said.

‘You wretched girl! Ungrateful runaway!’

‘Stop!’ Nico stepped forward with his sword drawn, but the spirit changed form and faced him.

This ghost was harder to see. She was a woman in an old-fashioned black velvet dress with a matching hat. She wore a string of pearls and white gloves, and her dark hair was tied back.

Nico stopped in his tracks. ‘No…’

‘My son,’ the ghost said. ‘I died when you were so young. I haunt the world in grief, wondering about you and your sister.’

‘Mama?’

‘No, it’s my mother,’ Thalia murmured, as if she still saw the first image.

My friends were helpless. The fog began thickening around their feet, twining around their legs like vines. The colours seemed to fade from their clothes and faces, as if they too had become shades.

‘Enough,’ I said, but my voice hardly worked. Despite the pain, I lifted my sword and stepped towards the ghost. ‘You’re not anybody’s mama!’

The ghost turned towards me. The image flickered, and I saw the goddess of ghosts in her true form.

You’d think after a while I would stop getting freaked out by the appearance of Greek ghoulies, but Melinoe caught me by surprise. Her right half was pale chalky white, like she’d been drained of blood. Her left half was pitch black and hardened like mummy skin. She wore a golden dress and a golden shawl. Her eyes were empty black voids and, when I looked into them, I felt as if I were seeing my own death.

‘Where are your ghosts?’ she demanded in irritation.

‘My… I don’t know. I don’t have any.’

She snarled. ‘Everyone has ghosts – deaths you regret. Guilt. Fear. Why can I not see yours?’

Thalia and Nico were still entranced, staring at the goddess as if she were their long-lost mother. I thought about other friends I’d seen die – Bianca di Angelo, Zoë Nightshade, Lee Fletcher, to name a few.

‘I’ve made my peace with them,’ I said. ‘They’ve passed on. They’re not ghosts. Now let my friends go!’

I slashed at Melinoe with my sword. She backed up quickly, growling in frustration. The fog dissipated around my friends. They stood blinking at the goddess as if they were now seeing how hideous she was.

‘What is that?’ Thalia said. ‘Where –’

‘It was a trick,’ Nico said. ‘She fooled us.’

‘You are too late, demigods,’ Melinoe said. Another petal fell off my carnation, leaving only one. ‘The deal has been struck.’

‘What deal?’ I demanded.

Melinoe made a hissing sound, and I realized it was her way of laughing. ‘So many ghosts, my young demigod. They long to be unleashed. When Kronos rules the world, I shall be free to walk among mortals both night and day, sowing terror as they deserve.’

‘Where’s the sword of Hades?’ I demanded. ‘Where’s Ethan?’

‘Close,’ Melinoe promised. ‘I will not stop you. I will not need to. Soon, Percy Jackson, you will have many ghosts. And you will remember me.’

Thalia notched an arrow and aimed it at the goddess. ‘If you open a path to the world, do you really think Kronos will reward you? He’ll cast you into Tartarus along with the rest of Hades’s servants.’

Melinoe bared her teeth. ‘Your mother was right, Thalia. You are an angry girl. Good at running away. Not much else.’

The arrow flew, but as it touched Melinoe she dissolved into fog, leaving nothing but the hiss of her laughter. Thalia’s arrow hit the rocks and shattered harmlessly.

‘Stupid ghost,’ she muttered.

I could tell she was really shaken up. Her eyes were rimmed with red. Her hands trembled. Nico looked just as stunned, like someone had smacked him between the eyes.

‘The thief…’ he managed. ‘Probably in the cave. We have to stop him before –’

Just then, the last petal fell off the carnation. The flower turned black and wilted.

‘Too late,’ I said.

A man’s laughter echoed down the mountain.

‘You’re right about that,’ a voice boomed. At the mouth of the cave stood two people – a boy with an eye patch and a three-metre-tall man in a tattered prison jumpsuit. The boy I recognized: Ethan Nakamura, son of Nemesis. In his hands was an unfinished sword – a double-edged blade of black Stygian iron with skeletal designs etched in silver. It had no hilt, but set in the base of the blade was a golden key, just like I’d seen in Persephone’s image. The key was glowing, as if Ethan had already invoked its power.

The giant man next to him had eyes of pure silver. His face was covered with a scraggly beard and his grey hair stuck out wildly. He looked thin and haggard in his ripped prison clothes, as though he’d spent the last few thousand years at the bottom of a pit, but even in this weakened state he looked plenty scary. He held out his hand and a giant spear appeared. I remembered what Thalia had said about Iapetus: His name means ‘the Piercer’ because that’s what he likes to do to his enemies.

The Titan smiled cruelly. ‘And now I will destroy you.’

‘Master!’ Ethan interrupted. He was dressed in combat fatigues with a backpack slung over his shoulder. His eye patch was crooked, his face smeared with soot and sweat. ‘We have the sword. We should –’

‘Yes, yes,’ the Titan said impatiently. ‘You’ve done well, Nawaka.’

‘It’s Nakamura, master.’

‘Whatever. I’m sure my brother Kronos will reward you. But now we have killing to attend to.’

‘My lord,’ Ethan persisted. ‘You’re not at full power. We should ascend and summon your brothers from the upper world. Our orders were to flee.’

The Titan whirled on him. ‘FLEE? Did you say FLEE?’

The ground rumbled. Ethan fell on his butt and scrambled backwards. The unfinished sword of Hades clattered to the rocks. ‘M-m-master, please –’

‘IAPETUS DOES NOT FLEE! I have waited three aeons to be summoned from the pit. I want revenge, and I will start by killing these weaklings!’

He levelled his spear at me and charged.

If he’d been at full strength, I have no doubt he would’ve pierced me right through the middle. Even weakened and just out of the pit, the guy was fast. He moved like a tornado, slashing so quickly I barely had time to dodge the strike before his spear impaled the rock where I’d been standing.

I was so dizzy I could barely hold my sword. Iapetus yanked the spear out of the rock, but as he turned to face me Thalia shot his flank full of arrows from his shoulder to his knee. He roared and turned on her, looking more angry than hurt. Ethan Nakamura tried to draw his own sword, but Nico yelled, ‘I don’t think so!’

The ground erupted in front of Ethan. Three armoured skeletons climbed out and engaged Ethan, pushing him back. The sword of Hades still lay on the rocks. If I could only get to it…

Iapetus slashed with his spear and Thalia leaped out of the way. She dropped her bow so she could draw her knives, but she wouldn’t last long in close combat.

Nico left Ethan to the skeletons and charged Iapetus. I was already ahead of him. It felt like my shoulder was going to explode, but I launched myself at the Titan and stabbed downward with Riptide, impaling the blade in the Titan’s calf.

‘AHHHH!’ Golden ichor gushed from the wound. Iapetus whirled and the shaft of his spear slammed into me, sending me flying.

I crashed into the rocks, right next to the River Lethe.

‘YOU DIE FIRST!’ Iapetus roared as he hobbled towards me. Thalia tried to get his attention by zapping him with an arc of electricity from her knives, but she might as well have been a mosquito. Nico stabbed with his sword but Iapetus knocked him aside without even looking. ‘I will kill you all! Then I will cast your souls into the eternal darkness of Tartarus!’

My eyes were full of spots. I could barely move. Another couple of centimetres and I would fall into the river headfirst.

The river.

I swallowed, hoping my voice still worked. ‘You’re – you’re even uglier than your son,’ I taunted the Titan. ‘I can see where Atlas gets his stupidity from.’

Iapetus snarled. He limped forward, raising his spear.

I didn’t know if I had the strength, but I had to try. Iapetus brought the spear down and I lurched sideways. The shaft impaled the ground right next to me. I reached up and grabbed his shirt collar, counting on the fact that he was off balance as well as hurt. He tried to regain his footing, but I pulled him forward with all my body weight. He stumbled and fell, grabbing my arms in a panic, and together we pitched into the Lethe.

FLOOOOOM! I was immersed in black water.

I prayed to Poseidon that my protection would hold and, as I sank to the bottom, I realized I was still dry. I knew my own name. And I still had the Titan by the shirt collar.

The current should’ve ripped him out of my hands, but somehow the river was channelling itself around me, leaving us alone.

With my last bit of strength, I climbed out of the river, dragging Iapetus with my good arm. We collapsed on the riverbank – me perfectly dry, the Titan dripping wet. His pure silver eyes were as big as moons.

Thalia and Nico stood over me in amazement. Up by the cave, Ethan Nakamura was just cutting down the last skeleton. He turned and froze when he saw his Titan ally spread eagled on the ground.

‘My – my lord?’ he called.

Iapetus sat up and stared at him. Then he looked at me and smiled.

‘Hello,’ he said. ‘Who am I?’

‘You’re my friend,’ I blurted out. ‘You’re… Bob.’

That seemed to please him greatly. ‘I am your friend Bob!’

Clearly, Ethan could tell things were not going his way. He glanced at the sword of Hades lying in the dirt, but before he could lunge for it, a silver arrow sprouted in the ground at his feet.