“Nico!” he yelled. “The scepter.”

Nico raised Diocletian’s scepter, and the cavern air shimmered purple. Ghosts climbed from the fissure and seeped from the walls—an entire Roman legion in full battle gear. They began taking on physical form, like walking corpses, but they seemed confused. Jason yelled in Latin, ordering them to form ranks and attack. The undead just shuffled among the monsters, causing momentary confusion, but that wouldn’t last.

Frank turned to Hazel and Leo. “You two keep going.”

Hazel’s eyes widened. “What? No!”

“You have to.” It was the hardest thing Frank had ever done, but he knew it was the only choice. “Find the Doors. Save Annabeth and Percy.”

“But—” Leo glanced over Frank’s shoulder. “Hit the deck!”

Frank dove for cover as a volley of rocks slammed overhead. When he managed to get up, coughing and covered in dust, the entrance to the tunnel was gone. An entire section of wall had collapsed, leaving a slope of smoking rubble.

“Hazel…” Frank’s voice broke. He had to hope she and Leo were alive on the other side. He couldn’t afford to think otherwise.

Anger swelled in his chest. He turned and charged toward the monster army.

FRANK WAS NO EXPERT ON GHOSTS, but the dead legionnaires must have all been demigods, because they were totally ADHD.

They clawed their way out of the pit, then milled about aimlessly, chest-bumping each other for no apparent reason, pushing one another back into the chasm, shooting arrows into the air as if trying to kill flies, and occasionally, out of sheer luck, throwing a javelin, a sword, or an ally in the direction of the enemy.

Meanwhile, the army of monsters got thicker and angrier. Earthborn threw volleys of stones that plowed into the zombie legionnaires, crushing them like paper. Female demons with mismatched legs and fiery hair (Frank guessed they were empousai) gnashed their fangs and shouted orders at the other monsters. A dozen Cyclopes advanced on the crumbling bridges, while seal-shaped humanoids—telkhines, like Frank had seen in Atlanta—lobbed vials of Greek fire across the chasm. There were even some wild centaurs in the mix, shooting flaming arrows and trampling their smaller allies under hoof. In fact, most of the enemy seemed to be armed with some kind of fiery weapon. Despite his new fireproof pouch, Frank found that extremely uncool.

He pushed through the crowd of dead Romans, shooting down monsters until his arrows were spent, slowly making his way toward his friends.

A little late, he realized—duh—he should turn into something big and powerful, like a bear or a dragon. As soon as the thought occurred, pain flared in his arm. He stumbled, looked down, and was astonished to find an arrow shaft protruding from his left biceps. His sleeve was soaked with blood.

The sight made him dizzy. Mostly it made him angry. He tried to turn into a dragon, with no luck. The pain made it too hard to focus. Maybe he couldn’t change shape while wounded.

Great, he thought. Now I find out.

He dropped his bow and picked up a sword from a fallen…well, he actually wasn’t sure what it was—some sort of reptilian lady warrior with snake trunks instead of legs. He slashed his way forward, trying to ignore the pain and the blood dripping down his arm.

About five meters ahead, Nico was swinging his black sword with one hand, holding the scepter of Diocletian aloft with the other. He kept shouting orders at the legionnaires, but they paid him no attention.

Of course not, Frank thought. He’s Greek.

Jason and Piper stood at Nico’s back. Jason summoned gusts of wind to blast aside javelins and arrows. He deflected a vial of Greek fire right up the throat of a gryphon, which burst into flames and spiraled into the pit. Piper put her new sword to good use, while spraying food from the cornucopia in her other hand—using hams, chickens, apples, and oranges as interceptor missiles. The air above the chasm turned into a fireworks show of flaming projectiles, exploding rocks, and fresh produce.

Still, Frank’s friends couldn’t hold out forever. Jason’s face was already beaded with sweat. He kept shouting in Latin: “Form ranks!” But the dead legionnaires wouldn’t listen to him, either. Some of the zombies were helpful just by standing in the way, blocking monsters and taking fire. If they kept getting mowed down, though, there wouldn’t be enough of them left to organize.

“Make way!” Frank shouted. To his surprise, the dead legionnaires parted for him. The closest ones turned and stared at him with blank eyes, as if waiting for further orders.

“Oh, great…” Frank mumbled.

In Venice, Mars had warned him that his true test of leadership was coming. Frank’s ghostly ancestor had urged him to take charge. But if these dead Romans wouldn’t listen to Jason, why should they listen to him? Because he was a child of Mars, or maybe because…

The truth hit him. Jason wasn’t quite Roman anymore. His time at Camp Half-Blood had changed him. Reyna had recognized that. Apparently, so did the undead legionnaires. If Jason no longer gave off the right sort of vibe, or the aura of a Roman leader…

Frank made it to his friends as a wave of Cyclopes crashed into them. He lifted his sword to parry a Cyclops’s club, then stabbed the monster in the leg, sending him backward into the pit. Another one charged. Frank managed to impale him, but blood loss was making him weak. His vision blurred. His ears rang.

He was dimly aware of Jason on his left flank, deflecting the incoming missiles with wind; Piper on his right, yelling charmspeak commands—encouraging the monsters to attack each other or take a refreshing jump into the chasm.

“It’ll be fun!” she promised.

A few listened, but across the pit, the empousai were countering her orders. Apparently they had charmspeak too. The monsters crowded so thickly around Frank that he could barely use his sword. The stench of their breath and body odor was almost enough to knock him out, even without the arrow throbbing in his arm.

What was Frank supposed to do? He’d had a plan, but his thoughts were getting fuzzy.

“Stupid ghosts!” Nico shouted.

“They won’t listen!” Jason agreed.

That was it. Frank had to make the ghosts listen.

He summoned all his strength and yelled, “Cohorts—lock shields!”

The zombies around him stirred. They lined up in front of Frank, putting their shields together in a ragged defensive formation. But they were moving too slowly, like sleepwalkers, and only a few had responded to his voice.

“Frank, how did you do that?” Jason yelled.

Frank’s head swam with pain. He forced himself not to pass out. “I’m the ranking Roman officer,” he said. “They—uh, they don’t recognize you. Sorry.”

Jason grimaced, but he didn’t look particularly surprised. “How can we help?”

Frank wished he had an answer. A gryphon soared overhead, almost decapitating him with its talons. Nico smacked it with the scepter of Diocletian, and the monster veered into a wall.

“Orbem formate!” Frank ordered.

About two dozen zombies obeyed, struggling to form a defensive ring around Frank and his friends. It was enough to give the demigods a little respite, but there were too many enemies pressing forward. Most of the ghostly legionnaires were still wandering around in a daze.

“My rank,” Frank realized.

“All these monsters are rank!” Piper yelled, stabbing a wild centaur.

“No,” Frank said. “I’m only a centurion.”

Jason cursed in Latin. “He means he can’t control a whole legion. He’s not of high enough rank.”

Nico swung his black sword at another gryphon. “Well, then, promote him!”

Frank’s mind was sluggish. He didn’t understand what Nico was saying. Promote him? How?

Jason shouted in his best drill-sergeant voice: “Frank Zhang! I, Jason Grace, praetor of the Twelfth Legion Fulminata, give you my final order: I resign my post and give you emergency field promotion to praetor, with the full powers of that rank. Take command of this legion!”

Frank felt as if a door had opened somewhere in the House of Hades, letting in a blast of fresh air that swept through the tunnels. The arrow in his arm suddenly didn’t matter. His thoughts cleared. His eyesight sharpened. The voices of Mars and Ares spoke in his mind, strong and unified: Break them!

Frank hardly recognized his own voice when he yelled: “Legion, agmen formate!”

Instantly, every dead legionnaire in the cavern drew his sword and raised his shield. They scrambled toward Frank’s position, pushing and hacking monsters out of their way until they stood shoulder to shoulder with the comrades, arranging themselves in a square formation. Stones, javelins, and fire rained down, but now Frank had a disciplined defensive line sheltering them behind a wall of bronze and leather.

“Archers!” Frank yelled. “Eiaculare flammas!”

He didn’t hold out much hope the command would work. The zombies’ bows couldn’t be in good shape. But to his surprise, several dozen ghostly skirmishers nocked arrows in unison. Their arrowheads caught fire spontaneously and a flaming wave of death arced over the legion’s line, straight into the enemy. Cyclopes fell. Centaurs stumbled. A telkhine shrieked and ran in circles with a burning arrow impaled in his forehead.

Frank heard a laugh behind him. He glanced back and couldn’t believe what he saw. Nico di Angelo was actually smiling.

“That’s more like it,” Nico said. “Let’s turn this tide!”

“Cuneum formate!” Frank yelled. “Advance with pila!”

The zombie line thickened in the center, forming a wedge designed to break through the enemy host. They lowered their spears in a bristling row and pushed forward.

Earthborn wailed and threw boulders. Cyclopes smashed their fists and clubs against the locked shields, but the zombie legionnaires were no longer paper targets. They had inhuman strength, hardly wavering under the fiercest attacks. Soon the floor was covered with monster dust. The line of javelins chewed through the enemy like a set of giant teeth, felling ogres and snake women and hellhounds. Frank’s archers shot gryphons out of the air and caused chaos in the main body of the monster army across the chasm.

Frank’s forces began to take control of their side of the cavern. One of the stone bridges collapsed, but more monsters kept pouring over the other one. Frank would have to stop that.

“Jason,” he called, “can you fly a few legionnaires across the pit? The enemy’s left flank is weak—see? Take it!”

Jason smiled. “With pleasure.”

Three dead Romans rose into the air and flew across the chasm. Then three more joined them. Finally Jason flew himself across and his squad began cutting through some very surprised-looking telkhines, spreading fear through the enemy’s ranks.

“Nico,” Frank said, “keep trying to raise the dead. We need more numbers.”

“On it.” Nico lifted the scepter of Diocletian, which glowed even darker purple. More ghostly Romans seeped from the walls to join the fight.

Across the chasm, empousai shouted commands in a language Frank didn’t know, but the gist was obvious. They were trying to shore up their allies and keep them charging across the bridge.