Rachel stood at the porch railing and sighed. She wore Bermuda shorts and her van Gogh T-shirt. (Yeah, Rachel was trying to teach me about art, but don't get too impressed. I only remembered the dude's name because he cut his ear off.)

I wondered if she was thinking about me, and how much it sucked that I wasn't with them on vacation. I know that's what I was thinking.

Then the scene changed. I was in St. Louis, standing downtown under the Arch. I'd been there before. In fact, I'd almost fallen to my death there before.

Over the city, a thunderstorm boiled—a wall of absolute black with lightning streaking across the sky. A few blocks away, swarms of emergency vehicles gathered with their lights flashing. A column of dust rose from a mound of rubble, which I realized was a collapsed skyscraper.

A nearby reporter was yelling into her microphone: "Officials are describing this as a structural failure, Dan, though no one seems to know if it is related to the storm conditions."

Wind whipped her hair. The temperature was dropping rapidly, like ten degrees just since I'd been standing there.

"Thankfully, the building had been abandoned for demolition," she said. "But police have evacuated all nearby buildings for fear the collapse might trigger—"

She faltered as a mighty groan cut through the sky. A blast of lightning hit the center of the darkness. The entire city shook. The air glowed, and every hair on my body stood up. The blast was so powerful I knew it could only be one thing: Zeus's master bolt. It should have vaporized its target, but the dark cloud only staggered backward. A smoky fist appeared out of the clouds. It smashed another tower, and the whole thing collapsed like children's blocks.

The reporter screamed. People ran through the streets. Emergency lights flashed. I saw a streak of silver in the sky—a chariot pulled by reindeer, but it wasn't Santa Claus driving. It was Artemis, riding the storm, shooting shafts of moonlight into the darkness. A fiery golden comet crossed her path . . . maybe her brother Apollo.

One thing was clear: Typhon had made it to the Mississippi River. He was halfway across the U.S., leaving destruction in his wake, and the gods were barely slowing him down.

The mountain of darkness loomed above me. A foot the size of Yankee Stadium was about to smash me when a voice hissed, "Percy!"

I lunged out blindly. Before I was fully awake, I had Nico pinned to the floor of the cell with the edge of my sword at his throat.

"Want . . . to . . . rescue," he choked.

Anger woke me up fast. "Oh, yeah? And why should I trust you?"

"No . . . choice?" he gagged.

I wished he hadn't said something logical like that. I let him go.

Nico curled into a ball and made retching sounds while his throat recovered. Finally he got to his feet, eyeing my sword warily. His own blade was sheathed. I suppose if he'd wanted to kill me, he could've done it while I slept. Still, I didn't trust him.

"We have to get out of here," he said.

"Why?" I said. "Does your dad want to talk to me again?"

He winced. "Percy, I swear on the River Styx, I didn't know what he was planning."

"You know what your dad is like!"

"He tricked me. He promised—" Nico held up his hands. "Look . . . right now, we need to leave. I put the guards to sleep, but it won't last."

I wanted to strangle him again. Unfortunately, he was right. We didn't have time to argue, and I couldn't escape on my own. He pointed at the wall. A whole section vanished, revealing a corridor.

"Come on." Nico led the way.

I wished I had Annabeth's invisibility hat, but as it turned out, I didn't need it. Every time we came to a skeleton guard, Nico just pointed at it, and its glowing eyes dimmed. Unfortunately, the more Nico did it, the more tired he seemed. We walked through a maze of corridors filled with guards. By the time we reached a kitchen staffed by skeletal cooks and servants, I was practically carrying Nico. He managed to put all the dead to sleep but nearly passed out himself. I dragged him out of the servants' entrance and into the Fields of Asphodel.

I almost felt relieved until I heard the sound of bronze gongs high in the castle.

"Alarms," Nico murmured sleepily.

"What do we do?"

He yawned then frowned like he was trying to remember. "How about . . . run?"

Running with a drowsy child of Hades was more like doing a three-legged race with a life-size rag doll. I lugged him along, holding my sword in front of me. The spirits of the dead made way like the Celestial bronze was a blazing fire.

The sound of gongs rolled across the fields. Ahead loomed the walls of Erebos, but the longer we walked, the farther away they seemed. I was about to collapse from exhaustion when I heard a familiar "WOOOOOF!"

Mrs. O'Leary bounded out of nowhere and ran circles around us, ready to play.

"Good girl.'" I said. "Can you give us a ride to the Styx?"

The word Styx got her excited. She probably thought I meant sticks. She jumped a few times, chased her tail just to teach it who was boss, and then calmed down enough for me to push Nico onto her back. I climb aboard, and she raced toward the gates. She leaped straight over the EZ-DEATH line, sending guards sprawling and causing more alarms to blare. Cerberus barked, but he sounded more excited than angry, like: Can I play too?

Fortunately, he didn't follow us, and Mrs. O'Leary kept running. She didn't stop until we were far upriver and the fires of Erebos had disappeared in the murk.

Nico slid off Mrs. O'Leary's back and crumpled in a heap on the black sand.

I took out a square of ambrosia—part of the emergency god-food I always kept with me. It was a little bashed up, but Nico chewed it.

"Uh," he mumbled. "Better."

"Your powers drain you too much," I noted.

He nodded sleepily. "With great power . . . comes great need to take a nap. Wake me up later."

"Whoa, zombie dude." I caught him before he could pass out again. "We're at the river. You need to tell me what to do."

I fed him the last of my ambrosia, which was a little dangerous. The stuff can heal demigods, but it can also burn us to ashes if we eat too much. Fortunately, it seemed to do the trick. Nico shook his head a few times and struggled to his feet.

"My father will be coming soon," he said. "We should hurry."

The River Styx's current swirled with strange objects—broken toys, ripped-up college diplomas, wilted homecoming corsages—all the dreams people had thrown away as they'd passed from life into death. Looking at the black water, I could think of about three million places I'd rather swim.

"So . . . I just jump in?"

"You have to prepare yourself first," Nico said, "or the river will destroy you. It will burn away your body and soul."

"Sounds fun," I muttered.

"This is no joke," Nico warned. "There is only one way to stay anchored to your mortal life. You have to . . ."

He glanced behind me and his eyes widened. I turned and found myself face-to-face with a Greek warrior.

For a second I thought he was Ares, because this guy looked exactly like the god of war—tall and buff, with a cruel scarred face and closely shaved black hair. He wore a white tunic and bronze armor. He held a plumed war helm under his arm. But his eyes were human—pale green like a shallow sea—and a bloody arrow stuck out of his left calf, just above the ankle.

I stunk at Greek names, but even I knew the greatest warrior of all time, who had died from a wounded heel.

"Achilles," I said.

The ghost nodded. "I warned the other one not to follow my path. Now I will warn you."

"Luke? You spoke with Luke?"

"Do not do this," he said. "It will make you powerful. But it will also make you weak. Your prowess in combat will be beyond any mortal's, but your weaknesses, your failings will increase as well."

"You mean I'll have a bad heel?" I said. "Couldn't I just, like, wear something besides sandals? No offense."

He stared down at his bloody foot. "The heel is only my physical weakness, demigod. My mother, Thetis, held me there when she dipped me in the Styx. What really killed me was my own arrogance. Beware! Turn back!"

He meant it. I could hear the regret and bitterness in his voice. He was honestly trying to save me from a terrible fate.

Then again, Luke had been here, and he hadn't turned back.

That's why Luke had been able to host the spirit of Kronos without his body disintegrating. This is how he'd prepared himself, and why he seemed impossible to kill. He had bathed in the River Styx and taken on the powers of the greatest mortal hero, Achilles. He was invincible.

"I have to," I said. "Otherwise I don't stand a chance."

Achilles lowered his head. "Let the gods witness I tried. Hero, if you must do this, concentrate on your mortal point. Imagine one spot of your body that will remain vulnerable. This is the point where your soul will anchor your body to the world. It will be your greatest weakness, but also your only hope. No man may be completely invulnerable. Lose sight of what keeps you mortal, and the River Styx will burn you to ashes. You will cease to exist."

"I don't suppose you could tell me Luke's mortal point?"

He scowled. "Prepare yourself, foolish boy. Whether you survive this or not, you have sealed your doom!"

With that happy thought, he vanished.

"Percy," Nico said, "maybe he's right."

"This was your idea."

"I know, but now that we're here—"

"Just wait on the shore. If anything happens to me . . . Well, maybe Hades will get his wish, and you'll be the child of the prophecy after all."

He didn't look pleased about that, but I didn't care.

Before I could change my mind, I concentrated on the small of my back—a tiny point just opposite my navel. It was well defended when I wore my armor. It would be hard to hit by accident, and few enemies would aim for it on purpose. No place was perfect, but this seemed right to me, and a lot more dignified than, like, my armpit or something.

I pictured a string, a bungee cord connecting me to the world from the small of my back. And I stepped into the river.

Imagine jumping into a pit of boiling acid. Now multiply that pain times fifty. You still won't be close to understanding what it felt like to swim in the Styx. I planned to walk in slow and courageous like a real hero. As soon as the water touched my legs, my muscles turned to jelly and I fell face-first into the current.

I submerged completely. For the first time in my life, I couldn't breathe underwater. I finally understood the panic of drowning. Every nerve in my body burned. I was dissolving in the water. I saw faces—Rachel, Grover, Tyson, my mother—but they faded as soon as they appeared.

"Percy," my mom said. "I give you my blessing."

"Be safe, brother!" Tyson pleaded.

"Enchiladas!" Grover said. I wasn't sure where that came from, but it didn't seem to help much.

I was losing the fight. The pain was too much. My hands and feet were melting into the water, my soul was being ripped from my body. I couldn't remember who I was. The pain of Kronos's scythe had been nothing compared to this.