Alright if you haven't read the title or the description, yes this is a Undertale AU except it's a HOO version of it! I love both of these two things so why not make an AU out of it?
Also you have know both of these topics to understand the story.
This first part of the fanfic will just be information cast and some minor changes.
Roles
Thalia replace Frisk role as the The Protagonist.
Hooty replace Flowy role as the empty one. (yes I know it's a TOH character but I couldn't come up with anything better so I'm sticking with that!)
Calypso replace Toriel role as the Caretaker.
Percy replace Sans role as the Judge (I mean there a ton of fanfic about Percy and the same with UT with Sans. So it makes sense for Percy to be Sans.)
Jason replace Papyrus role as the Ambitions one.
Nico replace Undyne role as The captain of the demi-guards
Hazel replace Napstablook role as the Recluse girl
Will replace Alphys role as the Royal Scientist/doctor. (Make total sense because the two are gay and in normal UT they're lesbos. So solangelo fans enjoy )
Frank replace Mettaton role as the celebrity role
Leo replace Asgore role as the Monarch king
Annabeth replace Asriel role as the Soulless Angel role
Renya replace Chara role as The Fallen child (might make an early appearance in the story. Not sure yet.)
Rachel replace Monster kid (Originally was going to be Luke, but I scrapped that idea for reasons)
Piper replace Muffet
And for who replace Gaster in this, well... let's say that it's going to be staying as a secret for now (And no, it's not Chiron. And yes there is a huge reason why it's not him).
Other minor role swap will be include during the book. Anyway onto some details of the story.
Details about the characters and other changes
-Story is still the same as before. Just with HOO characters.
-Thaila will look the same from before except she's not a demigod, but she will be having a bow with her. She still has her sliver bracelet on her and will have the same abilities as Frisk has like in the game. She and Jason aren't sibling in this.
-Underground is going to be called the Demi-grounds (because that's the best I can come up with right now)
- Hooty is a small brown barn owl. He can throw his feathers and use them as his attacks.
-The Ruins are a bit similar in UT but it's name has been change into where Calypso was being kept, Ogygia and it have more of a lush with life than in the Ruins.
-Calypso is the Caretaker of Ogygia. Her personality is mostly her's with a mix of Toriel's but she can act like how she is in when in TOA if you get to her badside. She came here to lay low from her boyfriend (Leo) after learning about out his plans.
-She likes weaving and making apple cider in her free time.
-Calypso still has her usually powers but she also inhibit Leo's fire power which she mostly use in battle (though she can't light herself on fire like him). She's also has grown taller to an young adult size, matching up with Leo's height.
-Her features are still all the same. She wears a long white robe with the Camp Half-blood Pegues symbol in it.
-The places are different in Ogygia from the places in the Ruins
-Hazel is as same as before, only difference is that she has more of faded ghostly appearance than normal her. She is still the daughter of Pluto's with all her abilities the same. She's and Nico are still half-siblings. She also use her gems, she makes, as swords.
-Snowdin and Snowdin forest was changed to Camp Half-blood and the forest that borders Camp Half-blood and some other places changes.
-Percy and Jason are half-brothers even though they aren't blood-related to each other and their fathers are completely different. But they reference to one another as brother.
-Percy is the younger shorter brother of Jason. His personality is mixed with his own and Sans personality. He likes to make puns, has a laid-back attitude, and eat blue cookies that his mom makes for him and he has a storage of them in hidden in his sentry station.
And when he's angry... well let's just say that there is always going to be a bad storm coming. His sea-green eyes also glow brightly on his right side when pissed.
-Percy is still the son of Poseidon and his abilities are still mostly the same but mixed in with Sans abilities (you'll see what I mean) and like how Sans uses Gaster blasters, Percy uses blasters call "God" blasters. (Every demigod God blaster will look like who their parents is. So Percy's god blaster is his dad, Poseidon, face instead of a skull.) Percy does still use Riptide in battle but he mostly focus on using his abilities and blaster instead.
-Percy features looks about the same, including of having his CHFB necklace, it just that he wears an dark navy blue hoodie along with his causal cloths
-Jason is still exactly like in the HOO series but he also has a bit of Papyrus personality as well and is older and taller than Percy. He's not as childish as Papyrus is but still embrace childish things and is pretty confident & intelligent but also oblivious. Jason is a very kind person who gives everybody a chance and accepts everyone for who they are even if they had done something bad in the past. He's also currently trying to get a position in Royal Demi-guard. Jason also happens to wear contact lens than glasses which he loses often.
-Jason wears a handmade orange shirt and armor with the camp half-blood words on it, black shorts, and has tied a short purple with gold trims cape with the words "SPQR" on the back.
-Jason abilities are pretty much the same and he's still a son of Jupiter/Zeus. He still has his flying ability to got to place quickly, but all of his attacks are electric. Jason does have his imperial gold gladius which he is able to change it to a spear, but like Percy, he mostly sticks with his abilities in battle situation.
-The waterfall happens to be the same.
- Nico doesn't have a lot of differences in this. His personality is the same and so is his features as HOO him and he's still angsty and moody as an emo, plus is still the son of Hades. He's always have a dark aura that follows him around. He take his work as working for the captain of the demi-guards very serious. His house happens to be his cabin which is behind the waterfall. He happens to be close to the same height as Jason is.
-Nico does happen to wear battle armor though, unlike his classic self. But it's design is like something you would see guarding Hades like his metal skull chestplate. Like how Undyne has her Undying Form, Nico also has an form when near-death called "Nico the Nightmare". It's a combination with changes in his metal bone armor and a Corpse Husband mask. He uses his Stygian Iron in battle and can summon throwable bones for attacks. Nico does get easily tired out after doing a major attack though.
- Nico in his causal outfit usually features a light-grey jacket, blue jeans, and a skull t-shirt.
-The Hotlands has been replace with New Rome.
-Where Alphys lab is usually is at has been change to the Apollo labs.
-Will generally the same as his HOO counterpart. He creates mostly medicine and do other health studies there. He wears a white lab coat over his orange t-shirt. Will does happen to have his bow with him, being the son of Apollo, but he use it only for self-defense. His body also has the ability to light up in the dark
-Will doesn't work in the labs just by himself, like Alphys. He also has his siblings working in there to, in places elsewhere in the lab.
-Will happens to have a crush on Nico, but doesn't know how the son of Hades will react if he tells him, so he usually keep it hidden in him and try's to ignore the crushing feeling by focusing on his work.
-Also Will happens to secretly enjoy anime.
-Frank also stays the same as himself in the role of Mettaton, like he's still a son of Mars and a divine ancestor of Poseidon. But he happen not only half-human but half-robot due to an freak accident which ending up making Will having to make some robotic parts into Frank. So though he seems to be an normal human demigod, his body is fully mechanical.
- Unlike how Mettaton is a confident, charismatic, and charming TV host that loves drama, action, and violence, Frank is more self conscious, kind, low-key, person. Frank does also happen to run a show, but it's a bit different from Mettaton's show as it usually involves different challenges of strength and ability test. Coincidentally, Frank still has a fear of fire.
- Whenever Frank shapeshifts into an animal as his gift of Mars, they all appeared as metal versions instead of flesh. Frank main weapon besides for his shapeshifting is his bow, which he uses often in battle.
-Where the New Home is supposed to be in the game, it's now New, New, New York City.
- Leo castle is actually design to look like the Argo 2 way.
- Leo is actually more mature and taller in this version of himself since he's in Asgore's role. While he still has the same Leo look and personality deep inside of himself, he locks it away due to the ton of weight and self-responsibility as the king of Demigods, making him change drastically. He enjoys tinkering and creating machine such as his pet dragon, Festus, during his free time. However he also holds onto a lot of pain on himself more then he did with his mom's death because of what's he been doing to help break free the demigods...
-Leo still has his ability to summon fire as the son of Hephaestus, and can still even light himself on fire without causing any harm on his body in a battle. His combat weapon consist of a bronze hammer which can expand itself.
-Like how Calypso wears an royal white robe, Leo wears an white cape with brown trims. Instead of wearing armor like Asgore does, Leo shows off his mechanic cloths with his magic belt on his waist and he has a pair of safety work googles strap around his head right below his crown.
- Annabeth is still an daughter of Athena, but she has conquer her fear of spiders, and she was living with Leo and Calypso. Her behavior is like how she was before knowing she was a demigod and meeting Luke and Thaila in the PJO series (so a bit shy and scared). She has Asriel docile, unadventurous, and loving child personality, mix with her intellectual genius, kind-hearted, wise and very witty nature. This was what she was like before being resurrected by Will without a soul as Hooty.
After that, she becomes incapable of feeling love, making her bored and resentful of everything.
-While alive, Annabeth features are the same, with her stormy-grey eyes and honey-color blond hair. She does have her CHFB necklace around her neck. She wears an orange long-sleeved shirt with yellow stripes, light blue shorts, and a brown belt at her waist.
-Even though living with Leo and Calypso, but not even related to them, she's very close to them and she thinks of them as adoptive-siblings/parents as they think of the same with her as well.
-Being smart isn't the only ability that Annabeth has anymore.
- Renya was the first human to fall into the Demi-grounds and to become friends with Annabeth. She happens to looks the same as before (of course) but her eyes are crimson red like Chara's.
- All of the HOO character ages are a bit different than like in Undertale. Most of them are teenagers or closed to teens as of Thaila being 12, Renya is 13, Annabeth is of 13, Percy is 17, Jason is 18, Nico is 18, Will is 17, Leo is the oldest out of the group of demigods with him being close to his 20s, Calypso happens to be 1 million or older than that, Piper and Rachel is the still the same age and Frank and Hazel don't have actually ages due to them being not alive.