These are changed events from the books. Hiccup is mentioned occasionally, and demigods from the Percy Jackson and Heroes of Olympus characters, such as Luke and Niko, that meet Hiccup. Also, I can't post regularly but will post new updates as I can. Anyways enjoy.
The Lightning Thief
"Hey Percy, check this out," Grover called to his friend. They were about to go to the underworld when Grover spotted a poster which said 'Will you take the plunge?' On the poster was a boy in a fur vest and a long sleeved green shirt. There was a crevice in the ground behind him.
"Who is that, Grover?" Percy asked his best friend.
"I remember reading about him," Annabeth began. "He was a demigod offered Elysium for slaying an evil dragon Queen but chose Tartarus to be with his best friend."
"Why would his best friend be in Tartarus?" Percy wondered.
"Because from what I remember, his best friend is a dragon." That made Percy look at Annabeth confused.
"How did that happen exactly? Don't all monsters kill demigods?"
Annabeth shrugged, "No idea. Guess something happened, and they became friends."
"Whatever. Let's keep going," Grover suggested, and they continued into the underworld.
Sea of Monsters
Luke was falling into Tartarus. The ground was approaching fast. All he could do was scream. As he was falling, something grabbed hold of his shoulders. Luke was taken safely to the ground, and when he looked up, he saw a boy riding on a dragon.
"You must be Luke," the boy said. "Kronos said you would be arriving soon, name's Hiccup." Hiccup held his hand out to greet Luke.
"You're the boy in Tartarus aren't you? But you looked different on the poster."
"When you live in Tartarus, you age really slowly," Hiccup stretched the vowel in really. "Anyways, I can get you to Kronos if you want a ride. Oh, and he also told me about the scar." Luke knew what he meant; a dragon gave him that scar. He considered the how long the journey would be if he walked, as well as the possible danger. Eventually, he decided to take the ride on the dragon.
"It wasn't easy to get this, Percy," Luke began. "I don't know what I would have done if it wasn't for Hiccup."
"Who?" Percy tilted his head to the side.
"The boy in Tartarus," Luke emphasized.
"Oh, why is he helping you?"
"He takes both sides into account," Luke said. "The reason Kronos killed his father was so he and his brothers wouldn't be thrown into the pit. And the reason he ate his children was so they couldn't overthrow him like he did to his father. Not like that worked out too well. Plus look what Zeus and all the other major gods did to the titans; they even hurt the ones who helped them. They also treat minor gods and their children badly. I think you may understand what I mean if you think about what you saw when you stayed in the Hermes cabin."
The Titans Curse
"So Agnes, where are you from?" Bianca asked one of her fellow Hunters. She wanted to know all about them.
"Well," Agnes began, "My full name is Agnes Jorgenson, and I am one of the oldest hunters here."
"How long, exactly, have you been a Hunter?" Bianca asked her friend.
"It's been about a Millennium, I think," Agnes answered, and Bianca looked at her in shock than said, "Why didn't we meet a few days ago when I took my history test?" Agnes chuckled at that.
"So, are you a demigod?" Bianca asked her friend.
"No but my uncle once removed is,"
"Who's your uncle?"
"You ever heard of Hiccup Haddock?"
"No."
"Well he is the first dragon rider. He died fighting an evil Dragon Queen. He and his Mighty Night Fury destroyed her but not without the cost of their lives. He taught my dad and a few other teens to train and ride dragons before his early death at age fifteen." Bianca gasped at the thought of one dying so young. "His and my Dad's first love interest, Astrid Hofferson, continued to teach them before she too, died a year later."
The Last Olympian
A thick green mist began to surround Rachel and she begins to speak
"Nine demigods shall answer the call,
To storm, fire, or night the world must fall,
Four misfits trapped in eternal night,
Shall once again return to the light,
An oath to keep the final breath,
And foes bear arms to the doors of death"
"What was that about?" Percy asked Chiron.
"She spoke the next great prophecy by the looks of it," he conjectured.
The Son of Neptune
"What's this one here?" Percy asked Octavian as he picked up a piece of paper.
"We call it the prophecy of 9," Octavian answered.
Mark of Athena
Niko was falling into Tartarus to try and close the Doors of death when something caught him. He was being held by a dragon. When the dragon landed, Niko got up to see his stygian iron blade was missing.
"I really wouldn't carry stygian iron here if I were you." Niko turned to the dragon and saw a person climb down from its back. This man was tall and slim with auburn hair and forest green eyes wearing a leather outfit. What surprised Niko was the strange black mist coming off of the boy.
"Who are you?" Niko asked the boy in front of him.
"Name's Hiccup Haddock, son of Nyx, and this here is my brother, Toothless," he gestured to the dragon, "And, yes, there both silly names, I know."
"I'm Niko Di Angelo, son of Hades." Niko announced. "I'm here to try and close the doors of Death. Could you possibly help me with that?"
"We'll talk at camp. Toothless, could you carry Niko?" Hiccup asked his brother who didn't respond well. "He says that as long as I hold on to your weapon, he will carry you there." Niko thought about it and gave Hiccup the blade which he placed in his backpack for safe keeping.
"But how will you get there?" Niko asked Hiccup as he mounted Toothless.
"Like this." Suddenly the black mist around Hicup began to shape itself into a pair of pitch black wings and tail, both identical to Toothless. "My mother taught me a few things in my time here," Hiccup said as he moved the wings around a bit. Then they all took to the skies.
Niko was sitting on the deck of the flying ship Argo II as they were heading to rescue Annabeth from the Spider Queen's Lair.
"I don't know if I would have survived if it wasn't for Hiccup"
"Who?" Frank asked
"The boy in Tartarus," Hazel answered. She remembered times when she thought about taking the plunge, but when the time came, she always chickened out. Without warning, a loud explosion went off. Everyone rushed to the side of the ship and looked down on the land below to see a collapsed parking lot with the Athena Parthenos in the midst of the rubble. A disheveled Annabeth was crawling from the rubble.
Percy yells to the crew to throw down a rope ladder. He descends and grabbed Annabeth from the rubble. "It's okay, we're together."
Percy explained everything that Happened with the Giants as they stood in the rubble. Then suddenly, the Cavern underneath the Parthenos began to collapse. Something grabbed Annabeth's bad ankle, and she realized she was being pulled into the pit. Percy hung on to Annabeth as she was pulled into Tartarus.
"We'll close the doors on this side." Percy yelled up to the crew.
"Hey Percy," Niko called down.
"Yeah Niko,"
"Tell Hiccup I said hello," Then Percy and Annabeth fell into the pit.
Sorry it took a while for me to write this. It was challenging to find spots in the series where events were changed. Plus, I need to have it spell-checked to prevent any spelling errors. I should be able to post sooner next time.