Frank nocked an arrow. He took aim, but the gryphon shrieked so loudly the sound echoed off the mountains. Frank flinched, and his shot went wide.

“I think that’s a call for help,” Percy warned. “We have to get out of here.”

With no clear plan, they ran for the docks. The gryphon dove after them. Percy slashed at it with his sword, but the gryphon veered out of reach.

They took the steps to the nearest pier and raced to the end. The gryphon swooped after them, its front claws extended for the kill. Hazel raised her sword, but an icy wall of water slammed sideways into the gryphon and washed it into the bay. The gryphon squawked and flapped its wings. It managed to scramble onto the pier, where it shook its black fur like a wet dog.

Frank grunted. “Nice one, Percy.”

“Yeah,” he said. “Didn’t know if I could still do that in Alaska. But bad news—look over there.” About a mile away, over the mountains, a black cloud was swirling—a whole flock of gryphons, dozens at least. There was no way they could fight that many, and no boat could take them away fast enough.

Frank nocked another arrow. “Not going down without a fight.”

Percy raised Riptide. “I’m with you.”

Then Hazel heard a sound in the distance—like the whinnying of a horse. She must’ve been imagining it, but she cried out desperately, “Arion! Over here!”

A tan blur came ripping down the street and onto the pier. The stallion materialized right behind the gryphon, brought down his front hooves, and smashed the monster to dust.

Hazel had never been so happy in her life. “Good horse! Really good horse!”

Frank backed up and almost fell off the pier. “How—?”

“He followed me!” Hazel beamed. “Because he’s the best—horse—EVER! Now, get on!”

“All three of us?” Percy said. “Can he handle it?”

Arion whinnied indignantly.

“All right, no need to be rude,” Percy said. “Let’s go.”

They climbed on, Hazel in front, Frank and Percy balancing precariously behind her. Frank wrapped his arms around her waist, and Hazel thought that if this was going to be her last day on earth—it wasn’t a bad way to go out.

“Run, Arion!” she cried. “To Hubbard Glacier!”

The horse shot across the water, his hooves turning the top of the sea to steam.

XLIII Hazel

RIDING ARION, HAZEL FELT POWERFUL, unstoppable, absolutely in control—a perfect combination of horse and human. She wondered if this was what it was like to be a centaur.

The boat captains in Seward had warned her it was three hundred nautical miles to the Hubbard Glacier, a hard, dangerous journey, but Arion had no trouble. He raced over the water at the speed of sound, heating the air around them so that Hazel didn’t even feel the cold. On foot, she never would have felt so brave. On horseback, she couldn’t wait to charge into battle.

Frank and Percy didn’t look so happy. When Hazel glanced back, their teeth were clenched and their eyeballs were bouncing around in their heads. Frank’s cheeks jiggled from the g-force. Percy sat in back, hanging on tight, desperately trying not to slip off the horse’s rear. Hazel hoped that didn’t happen. The way Arion was moving, she might not notice he was gone for fifty or sixty miles.

They raced through icy straits, past blue fjords and cliffs with waterfalls spilling into the sea. Arion jumped over a breaching humpback whale and kept galloping, startling a pack of seals off an iceberg.

It seemed like only minutes before they zipped into a narrow bay. The water turned the consistency of shaved ice in blue sticky syrup. Arion came to a halt on a frozen turquoise slab.

A half a mile away stood Hubbard Glacier. Even Hazel, who’d seen glaciers before, couldn’t quite process what she was looking at. Purple snowcapped mountains marched off in either direction, with clouds floating around their middles like fluffy belts. In a massive valley between two of the largest peaks, a ragged wall of ice rose out of the sea, filling the entire gorge. The glacier was blue and white with streaks of black, so that it looked like a hedge of dirty snow left behind on a sidewalk after a snowplow had gone by, only four million times as large.

As soon as Arion stopped, Hazel felt the temperature drop. All that ice was sending off waves of cold, turning the bay into the world’s largest refrigerator. The eeriest thing was a sound like thunder that rolled across the water.

“What is that?” Frank gazed at the clouds above the glacier. “A storm?”

“No,” Hazel said. “Ice cracking and shifting. Millions of tons of ice.”

“You mean that thing is breaking up?” Frank asked.

As if on cue, a sheet of ice silently calved off the side of the glacier and crashed into the sea, spraying water and frozen shrapnel several stories high. A millisecond later the sound hit them—a BOOM almost as jarring as Arion hitting the sound barrier.

“We can’t get close to that thing!” Frank said.

“We have to,” Percy said. “The giant is at the top.”

Arion nickered.

“Jeez, Hazel,” Percy said, “tell your horse to watch his language.”

Hazel tried not to laugh. “What did he say?”

“With the cussing removed? He said he can get us to the top.”

Frank looked incredulous. “I thought the horse couldn’t fly!”

This time Arion whinnied so angrily, even Hazel could guess he was cursing.

“Dude,” Percy told the horse, “I’ve gotten suspended for saying less than that. Hazel, he promises you’ll see what he can do as soon as you give the word.”

“Um, hold on, then, you guys,” Hazel said nervously. “Arion, giddyup!”

Arion shot toward the glacier like a runaway rocket, barreling straight across the slush like he wanted to play chicken with the mountain of ice.

The air grew colder. The crackling of the ice grew louder. As Arion closed the distance, the glacier loomed so large, Hazel got vertigo just trying to take it all in. The side was riddled with crevices and caves, spiked with jagged ridges like ax blades. Pieces were constantly crumbling off—some no larger than snowballs, some the size of houses.

When they were about fifty yards from the base, a thunderclap rattled Hazel’s bones, and a curtain of ice that would have covered Camp Jupiter calved away and fell toward them.

“Look out!” Frank shouted, which seemed a little unnecessary to Hazel.

Arion was way ahead of him. In a burst of speed, he zigzagged through the debris, leaping over chunks of ice and clambering up the face of the glacier.

Percy and Frank both cussed like horses and held on desperately while Hazel wrapped her arms around Arion’s neck. Somehow, they managed not to fall off as Arion scaled the cliffs, jumping from foothold to foothold with impossible speed and agility. It was like falling down a mountain in reverse.

Then it was over. Arion stood proudly at the top of a ridge of ice that loomed over the void. The sea was now three hundred feet below them.

Arion whinnied a challenge that echoed off the mountains. Percy didn’t translate, but Hazel was pretty sure Arion was calling out to any other horses that might be in the bay: Beat that, ya punks!

Then he turned and ran inland across the top of the glacier, leaping a chasm fifty feet across.

“There!” Percy pointed.

The horse stopped. Ahead of them stood a frozen Roman camp like a giant-sized ghastly replica of Camp Jupiter. The trenches bristled with ice spikes. The snow-brick ramparts glared blinding white. Hanging from the guard towers, banners of frozen blue cloth shimmered in the arctic sun.

There was no sign of life. The gates stood wide open. No sentries walked the walls. Still, Hazel had an uneasy feeling in her gut. She remembered the cave in Resurrection Bay where she’d worked to raise Alcyoneus—the oppressive sense of malice and the constant boom, boom, boom, like Gaea’s heartbeat. This place felt similar, as if the earth were trying to wake up and consume everything—as if the mountains on either side wanted to crush them and the entire glacier to pieces.

Arion trotted skittishly.

“Frank,” Percy said, “how about we go on foot from here?”

Frank sighed with relief. “Thought you’d never ask.”

They dismounted and took some tentative steps. The ice seemed stable, covered with a fine carpet of snow so that it wasn’t too slippery.

Hazel urged Arion forward. Percy and Frank walked on either side, sword and bow ready. They approached the gates without being challenged. Hazel was trained to spot pits, snares, trip lines, and all sorts of other traps Roman legions had faced for eons in enemy territory, but she saw nothing—just the yawning icy gates and the frozen banners crackling in the wind.

She could see straight down the Via Praetoria. At the crossroads, in front of the snow-brick principia, a tall, dark- robed figure stood, bound in icy chains.

“Thanatos,” Hazel murmured.

She felt as if her soul were being pulled forward, drawn toward Death like dust toward a vacuum. Her vision went dark. She almost fell off Arion, but Frank caught her and propped her up.

“We’ve got you,” he promised. “Nobody’s taking you away.”

Hazel gripped his hand. She didn’t want to let go. He was so solid, so reassuring, but Frank couldn’t protect her from Death. His own life was as fragile as a half-burned piece of wood.

“I’m all right,” she lied.

Percy looked around uneasily. “No defenders? No giant? This has to be a trap.”

“Obviously,” Frank said. “But I don’t think we have a choice.”

Before Hazel could change her mind, she urged Arion through the gates. The layout was so familiar—cohort barracks, baths, armory. It was an exact replica of Camp Jupiter, except three times as big. Even on horseback, Hazel felt tiny and insignificant, as if they were moving through a model city constructed by the gods.

They stopped ten feet from the robed figure.

Now that she was here, Hazel felt a reckless urge to finish the quest. She knew she was in more danger than when she’d been fighting the Amazons, or fending off the gryphons, or climbing the glacier on Arion’s back. Instinctively she knew that Thanatos could simply touch her, and she would die.

But she also had a feeling that if she didn’t see the quest through, if she didn’t face her fate bravely, she would still die—in cowardice and failure. The judges of the dead wouldn’t be lenient to her a second time.

Arion cantered back and forth, sensing her disquiet.

“Hello?” Hazel forced out the word. “Mr. Death?”

The hooded figure raised his head.

Instantly, the whole camp stirred to life. Figures in Roman armor emerged from the barracks, the principia, the armory, and the canteen, but they weren’t human. They were shades—the chattering ghosts Hazel had lived with for decades in the Fields of Asphodel. Their bodies weren’t much more than wisps of black vapor, but they managed to hold together sets of scale armor, greaves, and helmets. Frost-covered swords were strapped to their waists. Pila and dented shields floated in their smoky hands. The plumes on the centurions’ helmets were frozen and ragged. Most of the shades were on foot, but two soldiers burst out of the stables in a golden chariot pulled by ghostly black steeds.