Persephone sat on a silver throne and studied us. ‘If this were spring, I would be able to greet you properly in the world above. Alas, in winter this is the best I can do.’

She sounded bitter. After all these millennia, I guess she still resented living with Hades half the year. She looked so bleached and out-of-place, like an old photograph of springtime.

She turned towards me as if reading my thoughts. ‘Hades is my husband and master, young one. I would do anything for him. But in this case I need your help, and quickly. It concerns Lord Hades’s sword.’

Nico frowned. ‘My father doesn’t have a sword. He uses a staff in battle, and his helm of terror.’

‘He didn’t have a sword,’ Persephone corrected.

Thalia sat up. ‘He’s forging a new symbol of power? Without Zeus’s permission?’

The goddess of springtime pointed. Above the table, an image flickered to life: skeletal weapon-smiths worked over a forge of black flames, using hammers fashioned like metal skulls to beat a length of iron into a blade.

‘War with the Titans is almost upon us,’ Persephone said. ‘My lord Hades must be ready.’

‘But Zeus and Poseidon would never allow Hades to forge a new weapon!’ Thalia protested. ‘It would unbalance their power-sharing agreement.’

Persephone shook her head. ‘You mean it would make Hades their equal? Believe me, daughter of Zeus, the Lord of the Dead has no designs against his brothers. He knew they would never understand, however, which is why he forged the blade in secret.’

The image over the table shimmered. A zombie weapon-smith raised the blade, still glowing hot. Something strange was set in the base – not a gem. More like…

‘Is that a key?’ I asked.

Nico made a gagging sound. ‘The keys of Hades?’

‘Wait,’ Thalia said. ‘What are the keys of Hades?’

Nico’s face looked even paler than his stepmother’s. ‘Hades has a set of golden keys that can lock or unlock death. At least… that’s the legend.’

‘It is true,’ Persephone said.

‘How do you lock and unlock death?’ I asked.

‘The keys have the power to imprison a soul in the Underworld,’ Persephone said. ‘Or to release it.’

Nico swallowed. ‘If one of those keys has been set in the sword –’

‘The wielder can raise the dead,’ Persephone said, ‘or slay any living thing and send its soul to the Underworld with a mere touch of the blade.’

We were all silent. The shadowy fountain gurgled in the corner. Handmaidens floated around us, offering trays of fruit and candy that would keep us in the Underworld forever.

‘That’s a wicked sword,’ I said at last.

‘It would make Hades unstoppable,’ Thalia agreed.

‘So you see,’ Persephone said, ‘why you must help get it back.’

I stared at her. ‘Did you say get it back?’

Persephone’s eyes were beautiful and deadly serious, like poisonous blooms. ‘The blade was stolen when it was almost finished. I do not know how, but I suspect a demigod, some servant of Kronos. If the blade falls into the Titan lord’s hands –’

Thalia shot to her feet. ‘You allowed the blade to be stolen! How stupid was that? Kronos probably has it by now!’

Thalia’s arrows sprouted into long-stemmed roses. Her bow melted into a honeysuckle vine dotted with white and gold flowers.

‘Take care, huntress,’ Persephone warned. ‘Your father may be Zeus, and you may be the lieutenant of Artemis, but you do not speak to me with disrespect in my own palace.’

Thalia ground her teeth. ‘Give… me… back… my… bow.’

Persephone waved her hand. The bow and arrows changed back to normal. ‘Now, sit and listen. The sword could not have left the Underworld yet. Lord Hades used his remaining keys to shut down the realm. Nothing gets in or out until he finds the sword, and he is using all his power to locate the thief.’

Thalia sat down reluctantly. ‘Then what do you need us for?’

‘The search for the blade cannot be common knowledge,’ said the goddess. ‘We have locked the realm, but we have not announced why, nor can Hades’s servants be used for the search. They must not know the blade exists until it is finished. Certainly they can’t know it is missing.’

‘If they thought Hades was in trouble, they might desert him,’ Nico guessed. ‘And join the Titans.’

Persephone didn’t answer, but if a goddess can look nervous, she did. ‘The thief must be a demigod. No immortal can steal another immortal’s weapon directly. Even Kronos must abide by that Ancient Law. He has a champion down here somewhere. And to catch a demigod… we shall use three.’

‘Why us?’ I said.

‘You are the children of the three major gods,’ Persephone said. ‘Who could withstand your combined power? Besides, when you restore the sword to Hades, you will send a message to Olympus. Zeus and Poseidon will not protest against Hades’s new weapon if it is given to him by their own children. It will show that you trust Hades.’

‘But I don’t trust him,’ Thalia said.

‘Ditto,’ I said. ‘Why should we do anything for Hades, much less give him a super-weapon? Right, Nico?’

Nico stared at the table. His fingers tapped on his black Stygian blade.

‘Right, Nico?’ I prompted.

It took him a second to focus on me. ‘I have to do this, Percy. He’s my father.’

‘Oh, no way,’ Thalia protested. ‘You can’t believe this is a good idea!’

‘Would you rather have the sword in Kronos’s hands?’

He had a point there.

‘Time is wasting,’ Persephone said. ‘The thief may have accomplices in the Underworld, and he will be looking for a way out.’

I frowned. ‘I thought you said the realm was locked.’

‘No prison is airtight, not even the Underworld. Souls are always finding new ways out faster than Hades can close them. You must retrieve the sword before it leaves our realm, or all is lost.’

‘Even if we wanted to,’ Thalia said, ‘how would we find this thief?’

A potted plant appeared on the table: a sickly yellow carnation with a few green leaves. The flower listed sideways, as if it were trying to find the sun.

‘This will guide you,’ the goddess said.

‘A magic carnation?’ I asked.

‘The flower always faces the thief. As your prey gets closer to escaping, the petals will fall off.’

Right on cue, a yellow petal turned grey and fluttered to the ground.

‘If all the petals fall off,’ Persephone said, ‘the flower dies. This means the thief has reached an exit and you have failed.’

I glanced at Thalia. She didn’t seem too enthusiastic about the whole track-a-thief-with-a-flower thing. Then I looked at Nico. Unfortunately, I recognized the expression on his face. I knew what it was like wanting to make your dad proud, even if your dad was hard to love. In this case, really hard to love.

Nico was going to do this, with or without us. And I couldn’t let him go alone.

‘One condition,’ I told Persephone. ‘Hades will have to swear on the River Styx that he will never use this sword against the gods.’

The goddess shrugged. ‘I am not Lord Hades, but I am confident he would do this – as payment for your help.’

Another petal fell off the carnation.

I turned to Thalia. ‘I’ll hold the flower while you beat up the thief?’

She sighed. ‘Fine. Let’s go catch this jerk.’

The Underworld didn’t get into the Christmas spirit. As we made our way down the palace road into the Fields of Asphodel, it looked pretty much like it had on my previous visit – seriously depressing. Yellow grass and stunted black poplar trees rolled on forever. Shades drifted aimlessly across the hills, coming from nowhere and going nowhere, chattering to each other and trying to remember who they were in life. High above us, the cavern ceiling glistened darkly.

I carried the carnation, which made me feel pretty stupid. Nico led the way since his blade could clear a path through any crowd of undead. Thalia mostly grumbled that she should’ve known better than to go on a quest with a couple of boys.

‘Did Persephone seem kind of uptight?’ I asked.

Nico waded through a mob of ghosts, driving them back with Stygian iron. ‘She always acts that way when I’m around. She hates me.’

‘Then why did she include you in the quest?’

‘Probably my dad’s idea.’ He sounded like he wanted that to be true, but I wasn’t so sure.

It seemed strange to me that Hades hadn’t given us the quest himself. If this sword was so important to him, why had he let Persephone explain things? Usually Hades liked to threaten demigods in person.

Nico forged ahead. No matter how crowded the fields were – and if you’ve ever seen Times Square on New Year’s Eve, you’ll have a pretty good idea – the spirits parted before him.

‘He’s handy with zombie crowds,’ Thalia admitted. ‘Think I’ll take him along next time I go to the shopping mall.’

She gripped her bow tight, like she was afraid it would turn into a honeysuckle vine again. She didn’t look any older than she had last year, and it suddenly occurred to me that she would never age again now that she was a huntress. That meant I was older than her. Weird.

‘So,’ I said, ‘how’s immortality treating you?’

She rolled her eyes. ‘It’s not total immortality, Percy. You know that. We can still die in combat. It’s just… we don’t ever age or get sick, so we live forever, assuming we don’t get sliced to pieces by monsters.’

‘Always a danger.’

‘Always.’ She looked around, and I realized she was scanning the faces of the dead.

‘If you’re looking for Bianca,’ I said quietly, so Nico wouldn’t hear me, ‘she’d be in Elysium. She died a hero’s death.’

‘I know that,’ Thalia snapped. Then she caught herself. ‘It’s not that, Percy. I was just… never mind.’

A cold feeling washed over me. I remembered that Thalia’s mother had died in a car crash a few years ago. They’d never been close, but Thalia had never got to say goodbye. If her mother’s shade was wandering around down here – no wonder Thalia looked jumpy.

‘I’m sorry,’ I said. ‘I wasn’t thinking.’

Our eyes met, and I got the feeling she understood. Her expression softened. ‘It’s okay. Let’s just get this over with.’

Another petal fell off the carnation as we marched on.

I wasn’t happy when the flower pointed us towards the Fields of Punishment. I was hoping we’d veer into Elysium so we could hang out with the beautiful people and party, but no. The flower seemed to like the harshest, evillest part of the Underworld. We jumped over a lava stream and picked our way past scenes of horrible torture. I won’t describe them because you’d completely lose your appetite, but I wished I had cotton wool in my ears to shut out the screaming and the 1980s music.