Percy, Thalia, and Nico betrayed

Explaining

Percy pulled out the first two slips of paper. He sighed.
"Annabeth and Hazel," he said.
Annabeth groaned, and Hazel looked very upset. Frank and Piper tried their best to comfort Hazel, but no one came to Annabeth. 'Ha' Percy thought. 'Serves her right for being a b*tch.' Then he pulled out the next piece.
"Piper and Leo," he said, and Piper and Leo high-fived.
Jason looked angry, and Calypso just smiled. She knew Leo wouldn't cheat on her, he and Piper were best friends. Leo smiled at her, and Piper waved. Thalia smiled. They were good friends. Percy read the next paper, and upped Nico a little closer to him.
"Nico and Jason," he growled, and Nico cuddled closer to his boyfriend.
Jason smiled deviously, and Percy glared at him. Thalia felt a sudden protectiveness for Nico, and Leo just stared at his former best friend. Annabeth snickered, obviously, and she stood up and walked over to Jason. She whispered something in his ear, and Jason shook his head.
"I'm not doing that!" He yelled at her. Annabeth frowned, but walked away. When she sat down, Percy brought out the next slip.
"Thalia and Will," he said. Thalia rolled her eyes.
Will stood up and walked over to Thalia.
"Hey," he said nonchalantly. Thalia smacked him for his flirty facial expression.
Everyone laughed and snickered, and Will stalked to the far corner. Percy stopped laughing best he could and pulled out the next slip.
"And lastly, me and Frank," he said, and he and Frank smiled at each other.
"Okay guys! Now that everyone has a partner, we have five minutes to say the things we want to each other before the activities with your partner begin. Ready, and start!" Percy said and quickly ran to Nico. He pushed his blue and black bangs out of his eyes, and kissed Nico. He pulled back after a few seconds, and looked Nico in the eye.
"If he tries anything, and I mean anything like that, tell me. I don't care if it's against the rules. I need to know," Nico nodded.
"Don't worry so much. We used to be friends, I'll try to help him back to his normal self," Nico said back as stood on his tiptoes and pecked Percy on the cheek. Then he walked over to Jason and said a few words. Percy turned to Frank.
"Looks like we get some time to hang together!" He said excitedly. Frank chuckled.
'He's still the same dumb seaweed brain he always was' Frank thought as he watched Percy talk rapidly about what he wanted to do since he was gone so long. His ADHD was showing in his ramble, so Frank slapped his hand on Percy's mouth, shutting him up.
"Time started two minutes ago. Everyone's gone. Let's go!" He said, and grabbed Percy's arm and dragged him away.
After a few hours of doing things, and sparring for one of the hours, they sat down near Zeus' Fist. Frank was out of breath while Percy was normal, he hardly broke a sweat!
"I-didn't-know you-could fight-like-that still," Frank wheezed out, taking deep breaths as Percy handed him a water. Frank drank half the bottle in one gulp.
"Thanks. What was it like being banished, by the way?" Frank asked. Percy's face immediately frowned.
"When the gods banished us, they obviously siding care what happened. Other than a few who gave us their blessing. Therefore, we have new powers now. Since we couldn't use powers in Tartarus easily, we still have to train with them. But it actually wasn't so bad down there. I had this idea, and we went to this cool trash dump thing the gods use for their trash. We picked a few things and carried them back to our cave, and we each started building our own things. It was a training arena. We could practice sword fighting, sparring matches, obstacle courses, archery, and rock climbing the side of the mountain. We made a lot of friends, they all wanted to see our legendary training arena. We found a way to visit the Underworld, and we found a few friends we like to hang out with. You wouldn't know them, but their names are Silena, Luke, Beckendorf, and Bianca. We found a few others, but they liked doing their own things more," Percy finished, shrugging. Frank was awestruck.
"You did WHAT???" He yelled.
"What?!" Percy asked, confused at Frank's sudden outburst.
"A training arena, friends from Tartarus, making friends with people in the Underworld?? You know those people are dead, right?" Frank asked. Percy nodded.
"Yep. And they're our friends, got a problem with that?" Percy asked, snarling. Frank stood up and took a step back. Percy's loyalty was really showing for his friends.
Shadows started to form around Percy, and water from a fountain shot up in tremendous and frightening amounts and speed. Frank shook his frantically.
"N-no! Nope! Not at all!" He said. The water and shadows went back to their rightful place. Percy nodded.
"Good. Now let's go do the climbing wall!" Percy said, running towards the lava shooting tower. Frank sighed, and ran after him. Each step was asking the question more and more.
'What happened to Percy down there??'