AFTER THEIR FALL into Tartarus, jumping three hundred feet to the Mansion of Night should have felt quick.

Instead, Mia's heart seemed to slow down. Between the beats she had ample time to write her own obituary.

Amelia Starfury, died age 17.

BA-BOOM.

Died of massive injuries while leaping like an idiot into the abyss of Chaos and splattering on the entry hall floor of Nyx's mansion.

BA-BOOM.

Survived by her brother, sister, and maid / business partner, because everyone else she loved was dead. ( And even then, she didn't know if the former two were alive ).

BA-BOOM.

In lieu of flowers, please send donations to the Starfury Skyscraper, assuming Gaea hasn't already destroyed it.

Her feet hit solid floor. Pain shot up her legs, but she stumbled forward and broke into a run, hauling Annabeth and Percy after her.

Above them in the dark, Nyx and her children scuffled and yelled, "I've got them! My foot! Stop it!"

Mia kept running. She couldn't see anyway, so she closed her eyes. She used her other senses — listening for the echo of open spaces, feeling for cross-breezes against her face, sniffing for any scent of danger — smoke, or poison, or the stench of demons.

It wasn't the first time she'd plunged through darkness. After all, she'd practically lived in the Underworld, where it was always dark. But honestly the Underworld was like Cancun in comparison to this place.

The squabbling sounds of Nyx's children got farther away. That was good. She could feel Percy and Annabeth's life forces beside her. Also good.

In the distance ahead of them, Mia began to hear a throbbing sound, like her own heartbeat echoing back, amplified so powerfully, the floor vibrated underfoot. The sound filled her with dread, so she figured it must be the right way to go. She ran toward it.

As the beat got louder, she smelled smoke and heard the flickering of torches on either side. She guessed there would be light, but a crawling sensation across her neck warned her it would be a mistake to open her eyes.

"Don't look," Mia told her partners in crime.

"Wasn't planning on it," Percy said. "You both can feel that, right? We're still in the Mansion of Night. I do not want to see it."

Smart boy, Mia thought. His instincts were usually right on target. Even if his emotional intelligence wasn't as good as his instincts.

Whatever horrors lay in the Mansion of Night, they weren't meant for mortal eyes. Seeing them would be worse than staring at the face of Medusa. Better to run in darkness.

The throbbing got louder still, sending vibrations straight up Mia's spine. It felt like someone was knocking on the bottom of the world, demanding to be let in. She sensed the walls opening up on either side of them. The air smelled fresher — or at least not quite as sulfurous. There was another sound, too, closer than the deep pulsing . . . the sound of flowing water.

Mia's heart raced. She knew the exit was close. If they could make it out of the Mansion of Night, maybe they could leave the dark brood of demons behind.

But then her instincts told her that the water was too close for comfort, and she yanked Percy and Annabeth back so they wouldn't fall into the river.

* * *

"Annabeth!" Percy helped Mia pull her back, and Mia felt Annabeth's shoulder bury into her neck for the first time in . . . well, a long time.

"It's okay," Mia promised.

"Thanks . . ." The weight was off of Mia's shoulder. "Can you guys tell what's in front of us?"

"Water," Percy said. "I'm still not looking. I don't think it's safe yet."

"Agreed."

"I can sense a river," Mia reported. "Or maybe it's a moat. It's blocking our path, flowing left to right through a channel cut in the rock. The opposite side is about twenty feet away."

Annabeth let out a relieved noise. "Is there a bridge, or—?"

"I don't think so," Percy said. "And there's something wrong with the water. Listen."

Mia concentrated. Within the roaring current, thousands of voices cried out — shrieking in agony, pleading for mercy.

Help! they groaned. It was an accident!

The pain! their voices wailed. Make it stop!

Get me out of here, Your Highness!

Mia didn't need her eyes to see the river, she already knew what it looked like — a black briny current filled with tortured souls being swept deeper and deeper into Tartarus.

"The River Acheron," she breathed out. "The fifth river of the Underworld."

"I liked the Phlegethon better than this," Percy muttered.

"It's the River of Pain. The ultimate punishment for the souls of the damned — murderers, especially."

Murderers! the river wailed. Yes, like you!

Join us, another voice whispered. You are no better than we are.

Mia's head was flooded with images of all the monsters she'd killed over the years.

She resisted the urge to let out a whimper. Please don't, her mind pleaded.

But her mind betrayed her anyway — showing her Zoë Nightshade, who had been slain on Mount Tamalpais because she'd come to rescue Mia from the Titans. In vain, because she'd joined the Titans.

She saw Nico's sister, Bianca di Angelo, dying in the collapse of the metal giant Talos, because she also had tried to save Mia.

Michael Yew and Silena, gods, Silena . . . who had died in the Battle of Manhattan.

You could have prevented it, the river told Mia. You should have left Kronos. You should have done something.

Most painful of all: Sophia. Mia saw the knife flying and lodging into her head. Hell, she'd felt it, when she was cursed by the arai. She saw the sword stab into Sophia like it was scissors cutting into paper. Mia saw that happen again, Sophia's body lying on the ground, a pool of blood, hearing Gaea's laughter in the background.

Her blood is on your hands! the river wailed. You could have prevented her death. She could've been alive.

Stop it, Mia thought. Stop it, stop it, stop it. It was true, but stop. Please, stop.

You murdered her! the river cried. Jump in and feel what she felt! Make yourself equal to her.

Suddenly, hands gripped Mia and turned her body. "Mia," Percy's voice was firm. "Don't listen to it."

"I killed her, Delphinus." Mia let out a sob. "Do you think that I can forget that? How I let her down?"

"You can't forget that," Annabeth's voice quavered. "But Mia, we need you. The river isn't real. Sophia's okay, right?"

"Yeah . . ." Mia squeezed her eyes tightly once, then she forced the voices out of her head and focused on the task at hand. "This moat has to be the border of Night's territory. If we get across, we should be okay. I'm going to shadow-travel us."

"Do you have the strength for that?" Percy asked.

"I'm going to have to."

"Mia—" Annabeth started.

"There!" cried a voice behind them. "Kill the ungrateful tourists!"

The children of Nyx had found them. Mia grabbed onto Percy and Annabeth, concentrated on the other side — wherever that was — and they dissolved into shadow.

Then Mia felt that the ground was different under her feet. Her head was pounding, her ears were ringing.

"You can open your eyes," Percy said. "But you won't like what you see."

Mia slowly opened her eyes and blinked. After the darkness of Nyx, even the dim red glow of Tartarus seemed blinding.

Before them stretched a valley big enough to fit the Palace of Hades. The booming noise came from the entire landscape, as if thunder were echoing from beneath the ground. Under poisonous clouds, the rolling terrain glistened purple with dark red and blue scar lines.

"It looks like . . ." Annabeth sounded queasy. "Like a giant heart."

"The heart of Tartarus," Percy murmured.

The center of the valley was covered with a fine black fuzz of peppery dots. They were so far away, it took Mia a moment to realize she was looking at an army — thousands, maybe tens of thousands of monsters, gathered around a central pinpoint of darkness. It was too far to see any details, but Mia had no doubt what the pinpoint was. Even from the edge of the valley, she could feel its power tugging at her soul.

"The Doors of Death."

"Yeah." Percy's voice was hoarse. He still had the pale, wasted complexion of a corpse . . . which meant he looked about as good as Mia felt.

"What happened to Nyx . . .?" Annabeth asked.

Mia turned. They'd landed several hundred yards from the banks of Acheron, which flowed through a channel cut into black volcanic hills. Beyond that was nothing but darkness.

No sign of anyone coming after them. Apparently even the minions of Night didn't like to cross the Acheron.

Then, Mia heard the skittering of a rockslide in the hills to their left. Annabeth drew her drakon-bone sword. Percy raised Riptide. Mia raised her hand to her necklace.

A patch of glowing white hair appeared over the ridge, then a familiar grinning face with pure silver eyes.

"Bob?" Annabeth jumped up and down, grinning. "Oh my gods!"

"Friends!" The Titan lumbered toward them. The bristles of his broom had been burned off. His janitor's uniform was slashed with new claw marks, but he looked delighted. On his shoulder, Small Bob the kitten purred almost as loudly as the pulsing heart of Tartarus.

"I found you!" Bob gathered them in a rib-crushing hug. "You look like smoking dead people. That is good!"

"Urf," Percy said. "How did you get here? Through the Mansion of Night?"

"No, no." Bob shook his head adamantly. "That place is too scary. Another way — only good for Titans and such."

"Let me guess," Annabeth said. "You went sideways."

Bob scratched his chin, evidently at a loss for words. "Hmm. No. More . . . diagonal."

Mia laughed. Here they were at the heart of Tartarus, facing an impossible army — she would take any comfort she could get. She was ridiculously glad to have Bob the Titan with them again.

She kissed his immortal nose, which made him blink.

"We stay together now?" he asked.

"Yes," Annabeth agreed. "Time to see if this Death Mist works."

"And if it doesn't . . ." Percy stopped himself.

There was no point in wondering about that. They were about to march into the middle of an enemy army. If they were spotted, they were dead.

Despite that, Mia managed a smile. Their goal was in sight. They had a Titan with a broom and a very loud kitten on their side. That had to count for something.

"Doors of Death," she said, "here we come."