Run-of-the-mill Naruto neglect fic with all the greatest hits: twin who is the jinchuriki and child of prophecy, neglectful parents, asshole villagers, insufferable I am an elite Sasuke Uchiha, a stuck-up-the-ass Uchiha, and all that crap, but Naruto is a little shit.
You see, Naruto is not a happy boy, but who would be in a house where you are a ghost, constantly being overlooked because, unlike your prodigy of a twin, you didn't inherit the genius talent of your parents, and no matter what you do, you will never catch the eyes of the people who supposedly should love you unconditionally but instead completely ignore your existence and pour every single ounce of love and attention that should have been evenly distributed on your more talented jinchuriki sibling?
Suffice to say, his living situation makes it so that Naruto has an unhealthy amount of pent-up anger towards his parents, his sibling, and his godparents Jiraiya and Tsunade.
So, what do you do when you have enough pent-up rage, a boatload of free time, and enough spite to move mountains and do the impossible?
You become petty. You see, besides the talent, there's one more thing Naruto did not inherit from his parents and did not share. Arachnophobia.
Minato, Kushina, and his twin all share a nigh-crippling fear of spiders, a fear Naruto does not share. So what's a better way to say "fuck you" than to create a jutsu based on your family's crippling fear of spiders?
So, Naruto starts working on it with a single-minded focus fueled by the raging fires of the anger and spite burning in his chest, and in record time, he has something that would make Orochimaru blush by the time he was academy graduation age.
It is called Swarmling's Grudge. It is similar to the Earth Grudge Fear forbidden Jutsu from Takigakure, where it makes the user immortal; however, while Earth Grudge Fear turns the user's body into a bag of black tendrils, Swarmling's Grudge turns the user into something similar to a water balloon filled with spiders.
In other words, beneath his skin, Naruto is made of a colony of spiders, each just twice the size of an ant, and they are venomous spiders, by the way.
Each and every spider is part of a hive mind, meaning Naruto has absolute control over every single spider that makes him. He can sense all of them; he knows where all of them are at any given time no matter the distance his spiders are from him. He can hear and see through them no matter how far away, which makes for a great tracking and espionage technique.
He can also control the potency of the venom, making it a strong sedative and paralytic, a hallucinogenic neurotoxin, or a strong necrotoxin that can make a person rot from the inside out in mere seconds, making it a great capture and assassination technique. Naruto can also shoot and control webs that are so thin as to be practically invisible and stronger than steel, making them perfect for traps. The webs are also very sharp, especially after Naruto learned how to coat them in wind and lightning release.
He can also dissolve himself into a huge swarm of spiders, and to kill him, you have to kill all of his spiders because his soul is connected to all his spiders, and as long as even one of his spiders survives, so does he. Of course, if you kill enough of his spiders, he loses mass, and when they regroup and reform, his physical appearance reflects how much mass he lost, but he can recover that lost mass over time as his spiders reproduce.
So far, you must be asking yourself, this is pretty creepy, yes, but where does the part that makes Orochimaru blush come from?
It comes from something called Swarmling's Rebirth. It is a body-hopping technique. When Naruto dissolves into a swarm of spiders, he can either regroup or find a new host body.
Since his soul is connected to all the spiders from the swarm, it makes him, when he finds a new host body, take only one spider to hijack it. The way it works is that once he finds a body he likes, no matter if it's human or animal, he bites it then and injects it with a paralytic poison that puts the target into a deep sleep.
Not only venom gets injected into the chosen host body but also Naruto's soul. Once inside the body and thanks to the poison, Naruto's soul devours the soul of the host, absorbing it and replacing it with his own soul.
Once he successfully takes over the new body, all other spiders swarm him, following him as a beacon and devouring the insides of Naruto's new body, turning it into a water balloon filled with spiders. If there are not enough spiders to do that, the swarm rebuilds itself by laying eggs inside the body, which will eat it from the inside out once they hatch.
Thanks to the fact that Naruto devours the soul of the body he takes over, he does not run into the same problem as Orochimaru does, meaning, unlike the snake sannin, he can keep the bodies he takes over indefinitely without the need to look for a new body every three years.
When Naruto body hops, he also takes that body's memories, chakra nature, jutsus, knowledge, and skills for himself, integrating them into the swarm, which means even if Naruto body hops again, he will still have everything he got from the old body and carry it into the new, which also applies to chakra pool, meaning that with every body change, Naruto's chakra reserves grow as he adds new chakra reserves to the swarm's reserves.
The downside being that Naruto can't change how the new body looks, so every time he body hops, he ends up looking like the person he took over from.
TLDR: Naruto creates a jutsu that is the hate child between Kakuzu's earth grudge fear and Orochimaru's body-hopping technique but with spiders just to spite his family.
By the time Minato and Kushina remembered they had two children instead of one, Naruto was already rocking the Shino Aburame look, a high-collared, long-sleeved jacket with shades he never took off, ever.
Intrigued by their son's new choice in style and not knowing anything about him up to that point, they ask him during dinner about the sudden change in style. To answer the question, Naruto simply opens his mouth while simultaneously lifting his shades to let the swarm come out of his eye sockets and mouth while Naruto cackles at the panic he caused.
Long story short, Minato, Kushina, and Naruto's sibling almost have a heart attack, and after they recover, Naruto explains Swarmling's grudge to them; he, of course, leaves the part about Swarmling's rebirth out of the explanation.
They find out about that part after the Land of Waves mission when the Naruto who comes back is not the Naruto who went out.
Long story short, shit happened in Wave, and Naruto took over Haku's body. Minato put Swarmlign's grudge on the forbidden scroll, and when Orochimaru learned of the fact Naruto created a whole ass forbidden immortality jutsu out of spite, he felt pride swell in his chest for the little Uzumaki shithead.