ythologically speaking, if there’s anything I hate worse than trios of old ladies, it’s bulls.

Last summer, I fought the Minotaur on top of Half-Blood Hill. This time what I saw up there was even worse: two bulls. And not just regular bulls—bronze ones the size of elephants. And even that wasn’t bad enough. Naturally they had to breathe fire, too.

As soon as we exited the taxi, the Gray Sisters peeled out, heading back to New York, where life was safer. They didn’t even wait for their extra three-drachma payment. They just left us on the side of the road, Annabeth with nothing but her backpack and knife, Tyson and me still in our burned-up tie-dyed gym clothes.

“Oh, man,” said Annabeth, looking at the battle raging on the hill.

What worried me most weren’t the bulls themselves. Or the ten heroes in full battle armor who were getting their bronze-plated booties whooped. What worried me was that the bulls were ranging all over the hill, even around the back side of the pine tree. That shouldn’t have been possible. The camp’s magic boundaries didn’t allow monsters to cross past Thalia’s tree. But the metal bulls were doing it anyway.

One of the heroes shouted, “Border patrol, to me!” A girl’s voice—gruff and familiar.

Border patrol? I thought. The camp didn’t have a border patrol.

“It’s Clarisse,” Annabeth said. “Come on, we have to help her.”

Normally, rushing to Clarisse’s aid would not have been high on my “to do” list. She was one of the biggest bullies at camp. The first time we’d met she tried to introduce my head to a toilet. She was also a daughter of Ares, and I’d had a very serious disagreement with her father last summer, so now the god of war and all his children basically hated my guts.

Still, she was in trouble. Her fellow warriors were scattering, running in panic as the bulls charged. The grass was burning in huge swathes around the pine tree. One hero screamed and waved his arms as he ran in circles, the horsehair plume on his helmet blazing like a fiery Mohawk.

Clarisse’s own armor was charred. She was fighting with a broken spear shaft, the other end embedded uselessly in the metal joint of one bull’s shoulder.

I uncapped my ballpoint pen. It shimmered, growing longer and heavier until I held the bronze sword Anaklusmos in my hands. “Tyson, stay here. I don’t want you taking any more chances.”

“No!” Annabeth said. “We need him.”

I stared at her. “He’s mortal. He got lucky with the dodge balls but he can’t—”

“Percy, do you know what those are up there? The Colchis bulls, made by Hephaestus himself. We can’t fight them without Medea’s Sunscreen SPF 50,000. We’ll get burned to a crisp.”

“Medea’s what?”

Annabeth rummaged through her backpack and cursed. “I had a jar of tropical coconut scent sitting on my night-stand at home. Why didn’t I bring it?”

I’d learned a long time ago not to question Annabeth too much. It just made me more confused. “Look, I don’t know what you’re talking about, but I’m not going to let Tyson get fried.”

“Percy—”

“Tyson, stay back.” I raised my sword. “I’m going in.”

Tyson tried to protest, but I was already running up the hill toward Clarisse, who was yelling at her patrol, trying to get them into phalanx formation. It was a good idea. The few who were listening lined up shoulder-to-shoulder, locking their shields to form an ox-hide—and-bronze wall, their spears bristling over the top like porcupine quills.

Unfortunately, Clarisse could only muster six campers. The other four were still running around with their helmets on fire. Annabeth ran toward them, trying to help. She taunted one of the bulls into chasing her, then turned invisible, completely confusing the monster. The other bull charged Clarisse’s line.

I was halfway up the hill—not close enough to help. Clarisse hadn’t even seen me yet.

The bull moved deadly fast for something so big. Its metal hide gleamed in the sun. It had fist-sized rubies for eyes, and horns of polished silver. When it opened its hinged mouth, a column of white-hot flame blasted out.

“Hold the line!” Clarisse ordered her warriors.

Whatever else you could say about Clarisse, she was brave. She was a big girl with cruel eyes like her father’s. She looked like she was born to wear Greek battle armor, but I didn’t see how even she could stand against that bull’s charge.

Unfortunately, at that moment, the other bull lost interest in finding Annabeth. It turned, wheeling around behind Clarisse on her unprotected side.

“Behind you!” I yelled. “Look out!”

I shouldn’t have said anything, because all I did was startle her. Bull Number One crashed into her shield, and the phalanx broke. Clarisse went flying backward and landed in a smoldering patch of grass. The bull charged past her, but not before blasting the other heroes with its fiery breath. Their shields melted right off their arms. They dropped their weapons and ran as Bull Number Two closed in on Clarisse for the kill.

I lunged forward and grabbed Clarisse by the straps of her armor. I dragged her out of the way just as Bull Number Two freight-trained past. I gave it a good swipe with Riptide and cut a huge gash in its flank, but the monster just creaked and groaned and kept on going.

It hadn’t touched me, but I could feel the heat of its metal skin. Its body temperature could’ve microwaved a frozen burrito.

“Let me go!” Clarisse pummeled my hand. “Percy, curse you!”

I dropped her in a heap next to the pine tree and turned to face the bulls. We were on the inside slope of the hill now, the valley of Camp Half-Blood directly below us—the cabins, the training facilities, the Big House—all of it at risk if these bulls got past us.

Annabeth shouted orders to the other heroes, telling them to spread out and keep the bulls distracted.

Bull Number One ran a wide arc, making its way back toward me. As it passed the middle of the hill, where the invisible boundary line should’ve kept it out, it slowed down a little, as if it were struggling against a strong wind; but then it broke through and kept coming. Bull Number Two turned to face me, fire sputtering from the gash I’d cut in its side. I couldn’t tell if it felt any pain, but its ruby eyes seemed to glare at me like I’d just made things personal.

I couldn’t fight both bulls at the same time. I’d have to take down Bull Number Two first, cut its head off before Bull Number One charged back into range. My arms already felt tired. I realized how long it had been since I’d worked out with Riptide, how out of practice I was.

I lunged but Bull Number Two blew flames at me. I rolled aside as the air turned to pure heat. All the oxygen was sucked out of my lungs. My foot caught on something—a tree root, maybe—and pain shot up my ankle. Still, I managed to slash with my sword and lop off part of the monster’s snout. It galloped away, wild and disoriented. But before I could feel too good about that, I tried to stand, and my left leg buckled underneath me. My ankle was sprained, maybe broken.

Bull Number One charged straight toward me. No way could I crawl out of its path.

Annabeth shouted: “Tyson, help him!”

Somewhere near, toward the crest of the hill, Tyson wailed, “Can’t—get—through!”

“I, Annabeth Chase, give you permission to enter camp!”

Thunder shook the hillside. Suddenly Tyson was there, barreling toward me, yelling: “Percy needs help!”

Before I could tell him no, he dove between me and the bull just as it unleashed a nuclear firestorm.

“Tyson!” I yelled.

The blast swirled around him like a red tornado. I could only see the black silhouette of his body. I knew with horrible certainty that my friend had just been turned into a column of ashes.

But when the fire died, Tyson was still standing there, completely unharmed. Not even his grungy clothes were scorched. The bull must’ve been as surprised as I was, because before it could unleash a second blast, Tyson balled his fists and slammed them into the bull’s face. “BAD COW!”

His fists made a crater where the bronze bull’s snout used to be. Two small columns of flame shot out of its ears. Tyson hit it again, and the bronze crumpled under his hands like aluminum foil.

The bull’s face now looked like a sock puppet pulled inside out.

“Down!” Tyson yelled.

The bull staggered and fell on its back. Its legs moved feebly in the air, steam coming out of its ruined head in odd places.

Annabeth ran over to check on me.

My ankle felt like it was filled with acid, but she gave me some Olympian nectar to drink from her canteen, and I immediately started to feel better. There was a burning smell that I later learned was me. The hair on my arms had been completely singed off.

“The other bull?” I asked.

Annabeth pointed down the hill. Clarisse had taken care of Bad Cow Number Two. She’d impaled it through the back leg with a celestial bronze spear. Now, with its snout half gone and a huge gash in its side, it was trying to run in slow motion, going in circles like some kind of merry-go-round animal.

Clarisse pulled off her helmet and marched toward us. A strand of her stringy brown hair was smoldering, but she didn’t seem to notice. “You—ruin—everything!” she yelled at me. “I had it under control!”

I was too stunned to answer. Annabeth grumbled, “Good to see you too, Clarisse.”

“Argh!” Clarisse screamed. “Don’t ever, EVER try saving me again!”

“Clarisse,” Annabeth said, “you’ve got wounded campers.”

That sobered her up. Even Clarisse cared about the soldiers under her command.

“I’ll be back,” she growled, then trudged off to assess the damage.

I stared at Tyson. “You didn’t die.”

Tyson looked down like he was embarrassed. “I am sorry. Came to help. Disobeyed you.”

“My fault,” Annabeth said. “I had no choice. I had to let Tyson cross the boundary line to save you. Otherwise, you would’ve died.”

“Let him cross the boundary line?’” I asked. “But—”

“Percy,” she said, “have you ever looked at Tyson closely? I mean … in the face. Ignore the Mist, and really look at him.”

The Mist makes humans see only what their brains can process … I knew it could fool demigods too, but…

I looked Tyson in the face. It wasn’t easy. I’d always had trouble looking directly at him, though I’d never quite understood why. I’d thought it was just because he always had peanut butter in his crooked teeth. I forced myself to focus at his big lumpy nose, then a little higher at his eyes.

No, not eyes.

One eye. One large, calf-brown eye, right in the middle of his forehead, with thick lashes and big tears trickling down his cheeks on either side.

“Tyson,” I stammered. “You’re a …”

“Cyclops,” Annabeth offered. “A baby, by the looks of him. Probably why he couldn’t get past the boundary line as easily as the bulls. Tyson’s one of the homeless orphans.”

“One of the what?”

“They’re in almost all the big cities,” Annabeth said distastefully. “They’re … mistakes, Percy.

Children of nature spirits and gods … Well, one god in particular, usually … and they don’t always come out right. No one wants them. They get tossed aside. They grow up wild on the streets. I don’t know how this one found you, but he obviously likes you. We should take him to Chiron, let him decide what to do.”

“But the fire. How—”

“He’s a Cyclops.” Annabeth paused, as if she were remembering something unpleasant.

“They work the forges of the gods. They have to be immune to fire. That’s what I was trying to tell you.”

I was completely shocked. How had I never realized what Tyson was?

But I didn’t have much time to think about it just then. The whole side of the hill was burning.

Wounded heroes needed attention. And there were still two banged-up bronze bulls to dispose of, which I didn’t figure would fit in our normal recycling bins.

Clarisse came back over and wiped the soot off her forehead. “Jackson, if you can stand, get up. We need to carry the wounded back to the Big House, let Tantalus know what’s happened.”

“Tantalus?” I asked.

“The activities director,” Clarisse said impatiently.

“Chiron is the activities director. And where’s Argus? He’s head of security. He should be here.”

Clarisse made a sour face. “Argus got fired. You two have been gone too long. Things are changing.”

“But Chiron … He’s trained kids to fight monsters for over three thousand years. He can’t just be gone. What happened?”

”That happened,” Clarisse snapped.

She pointed to Thalia’s tree.

Every camper knew the story behind the tree. Six years ago, Grover, Annabeth, and two other demigods named Thalia and Luke had come to Camp Half-Blood chased by an army of monsters. When they got cornered on top of this hill, Thalia, a daughter of Zeus, had made her last stand here to give her friends time to reach safety. As she was dying, her father, Zeus, took pity on her and changed her into a pine tree. Her spirit had reinforced the magic borders of the camp, protecting it from monsters. The pine had been here ever since, strong and healthy.

But now, its needles were yellow. A huge pile of dead ones littered the base of the tree. In the center of the trunk, three feet from the ground, was a puncture mark the size of a bullet hole, oozing green sap.

A sliver of ice ran through my chest. Now I understood why the camp was in danger. The magical borders were failing because Thalia’s tree was dying.

Someone had poisoned it.