If Hazel woke up the rest of the crew every time they had a problem, they’d just start arguing again, feeling more and more hopeless.

She had to make Percy and Annabeth proud of her. She had to take the initiative. She couldn’t believe her only role in this quest would be what Nico had warned her of—removing the obstacle waiting for them in the House of Hades. She pushed the thought aside.

“We need some creative thinking,” she said. “Another way to cross those mountains, or a way to hide ourselves from the numina.”

Nico sighed. “If I was on my own, I could shadow-travel. But that won’t work for an entire ship. And honestly, I’m not sure I have the strength to even transport myself anymore.”

“I could maybe rig some kind of camouflage,” Leo said, “like a smoke screen to hide us in the clouds.” He didn’t sound very enthusiastic.

Hazel stared down at the rolling farmland, thinking about what lay beneath it—the realm of her father, lord of the Underworld. She’d only met Pluto once, and she hadn’t even realized who he was. She certainly had never expected help from him—not when she was alive the first time, not during her time as a spirit in the Underworld, not since Nico had brought her back to the world of the living.

Her dad’s servant Thanatos, god of death, had suggested that Pluto might be doing Hazel a favor by ignoring her. After all, she wasn’t supposed to be alive. If Pluto took notice of her, he might have to return her to the land of the dead.

Which meant calling on Pluto would be a very bad idea. And yet…

Please, Dad, she found herself praying. I have to find a way to your temple in Greece—the House of Hades. If you’re down there, show me what to do.

At the edge of the horizon, a flicker of movement caught her eye—something small and beige racing across the fields at incredible speed, leaving a vapor trail like a plane’s.

Hazel couldn’t believe it. She didn’t dare hope, but it had to be…“Arion.”

“What?” Nico asked.

Leo let out a happy whoop as the dust cloud got closer. “It’s her horse, man! You missed that whole part. We haven’t seen him since Kansas!”

Hazel laughed—the first time she’d laughed in days. It felt so good to see her old friend.

About a mile to the north, the small beige dot circled a hill and stopped at the summit. He was difficult to make out, but when the horse reared and whinnied, the sound carried all the way to the Argo II. Hazel had no doubt—it was Arion.

“We have to meet him,” she said. “He’s here to help.”

“Yeah, okay.” Leo scratched his head. “But, uh, we talked about not landing the ship on the ground anymore, remember? You know, with Gaea wanting to destroy us and all.”

“Just get me close, and I’ll use the rope ladder.” Hazel’s heart was pounding. “I think Arion wants to tell me something.”

HAZEL HAD NEVER FELT SO HAPPY. Well, except for maybe on the night of the victory feast at Camp Jupiter, when she’d kissed Frank for the first time…but this was a close second.

As soon as she reached the ground, she ran to Arion and threw her arms around him. “I missed you!” She pressed her face into the horse’s warm neck, which smelled of sea salt and apples. “Where have you been?”

Arion nickered. Hazel wished she could speak Horse like Percy could, but she got the general idea. Arion sounded impatient, as if saying, No time for sentiment, girl! Come on!

“You want me to go with you?” she guessed.

Arion bobbed his head, trotting in place. His dark brown eyes gleamed with urgency.

Hazel still couldn’t believe he was actually here. He could run across any surface, even the sea; but she’d been afraid he wouldn’t follow them into the ancient lands. The Mediterranean was too dangerous for demigods and their allies.

He wouldn’t have come unless Hazel was in dire need. And he seemed so agitated.… Anything that could make a fearless horse skittish should have terrified Hazel.

Instead, she felt elated. She was so tired of being seasick and airsick. Aboard the Argo II, she felt about as useful as a box of ballast. She was glad to be back on solid ground, even if it was Gaea’s territory. She was ready to ride.

“Hazel!” Nico called down from the ship. “What’s going on?”

“It’s fine!” She crouched down and summoned a gold nugget from the earth. She was getting better at controlling her power. Precious stones hardly ever popped up around her by accident anymore, and pulling gold from the ground was easy.

She fed Arion the nugget…his favorite snack. Then she smiled up at Leo and Nico, who were watching her from the top of the ladder a hundred feet above. “Arion wants to take me somewhere.”

The boys exchanged nervous looks.

“Uh…” Leo pointed north. “Please tell me he’s not taking you into that?”

Hazel had been so focused on Arion, she hadn’t noticed the disturbance. A mile away, on the crest of the next hill, a storm had gathered over some old stone ruins—maybe the remains of a Roman temple or a fortress. A funnel cloud snaked its way down toward the hill like an inky black finger.

Hazel’s mouth tasted like blood. She looked at Arion. “You want to go there?”

Arion whinnied, as if to say, Uh, duh!

Well…Hazel had asked for help. Was this her dad’s answer?

She hoped so, but she sensed something besides Pluto at work in that storm…something dark, powerful, and not necessarily friendly.

Still, this was her chance to help her friends—to lead instead of follow.

She tightened the straps of her Imperial gold cavalry sword and climbed onto Arion’s back.

“I’ll be okay!” she called up to Nico and Leo. “Stay put and wait for me.”

“Wait for how long?” Nico asked. “What if you don’t come back?”

“Don’t worry, I will,” she promised, hoping it was true.

She spurred Arion, and they shot across the countryside, heading straight for the growing tornado.

THE STORM SWALLOWED THE HILL in a swirling cone of black vapor.

Arion charged straight into it.

Hazel found herself at the summit, but it felt like a different dimension. The world lost its color. The walls of the storm encircled the hill in murky black. The sky churned gray. The crumbling ruins were bleached so white, they almost glowed. Even Arion had turned from caramel brown to a dark shade of ash.

In the eye of the tempest, the air was still. Hazel’s skin tingled coolly, as if she’d been rubbed with alcohol. In front of her, an arched gateway led through mossy walls into some sort of enclosure.

Hazel couldn’t see much through the gloom, but she felt a presence within, as if she were a chunk of iron close to a large magnet. Its pull was irresistible, dragging her forward.

Yet she hesitated. She reined in Arion, and he clopped impatiently, the ground crackling under his hooves. Wherever he stepped, the grass, dirt, and stones turned white like frost. Hazel remembered the Hubbard Glacier in Alaska—how the surface had cracked under their feet. She remembered the floor of that horrible cavern in Rome crumbling to dust, plunging Percy and Annabeth into Tartarus.

She hoped this black-and-white hilltop wouldn’t dissolve under her, but she decided it was best to keep moving.

“Let’s go, then, boy.” Her voice sounded muffled, as if she were speaking into a pillow.

Arion trotted through the stone archway. Ruined walls bordered a square courtyard about the size of a tennis court. Three other gateways, one in the middle of each wall, led north, east, and west. In the center of the yard, two cobblestone paths intersected, making a cross. Mist hung in the air—hazy shreds of white that coiled and undulated as if they were alive.

Not mist, Hazel realized. The Mist.

All her life, she’d heard about the Mist—the supernatural veil that obscured the world of myth from the sight of mortals. It could deceive humans, even demigods, into seeing monsters as harmless animals, or gods as regular people.

Hazel had never thought of it as actual smoke, but as she watched it curling around Arion’s legs, floating through the broken arches of the ruined courtyard, the hairs stood up on her arms. Somehow she knew: this white stuff was pure magic.

In the distance, a dog howled. Arion wasn’t usually scared of anything, but he reared, huffing nervously.

“It’s okay.” Hazel stroked his neck. “We’re in this together. I’m going to get down, all right?”

She slid off Arion’s back. Instantly he turned and ran.

“Arion, wai—”

But he’d already disappeared the way he’d come.

So much for being in this together.

Another howl cut through the air—closer this time.

Hazel stepped toward the center of the courtyard. The Mist clung to her like freezer fog.

“Hello?” she called.

“Hello,” a voice answered.

The pale figure of a woman appeared at the northern gateway. No, wait…she stood at the eastern entrance. No, the western. Three smoky images of the same woman moved in unison toward the center of the ruins. Her form was blurred, made from Mist, and she was trailed by two smaller wisps of smoke, darting at her heels like animals. Some sort of pets?

She reached the center of the courtyard and her three forms merged into one. She solidified into a young woman in a dark sleeveless gown. Her golden hair was gathered into a high-set ponytail, Ancient Greek style. Her dress was so silky, it seemed to ripple, as if the cloth were ink spilling off her shoulders. She looked no more than twenty, but Hazel knew that meant nothing.

“Hazel Levesque,” said the woman.

She was beautiful, but deathly pale. Once, back in New Orleans, Hazel had been forced to attend a wake for a dead classmate. She remembered the lifeless body of the young girl in the open casket. Her face had been made up prettily, as if she were resting, which Hazel had found terrifying.

This woman reminded Hazel of that girl—except the woman’s eyes were open and completely black. When she tilted her head, she seemed to break into three different people again…misty afterimages blurring together, like a photograph of someone moving too fast to capture.

“Who are you?” Hazel’s fingers twitched at the hilt of her sword. “I mean…which goddess?”

Hazel was sure of that much. This woman radiated power. Everything around them—the swirling Mist, the monochromatic storm, the eerie glow of the ruins—was because of her presence.

“Ah.” The woman nodded. “Let me give you some light.”

She raised her hands. Suddenly she was holding two old-fashioned reed torches, guttering with fire. The Mist receded to the edges of the courtyard. At the woman’s sandaled feet, the two wispy animals took on solid form. One was a black Labrador retriever. The other was a long, gray, furry rodent with a white mask around its face. A weasel, maybe?

The woman smiled serenely.

“I am Hecate,” she said. “Goddess of magic. We have much to discuss if you’re to live through tonight.”

HAZEL WANTED TO RUN, but her feet seemed stuck to the white-glazed ground.