Perhaps it was created by the Sage of Six Paths. Since he had a great influence on the Naruto world, it's possible he made this sword with special powers. And as Naruto is related to the Sage of Six Paths in some way, he is the one fated to wield it in the fanfiction. The sword might have been hidden away for centuries until Naruto's time.
In some fanfictions, the sword could be a gift from another dimension. There could be a portal that opened up and the sword came through. It was meant for a great hero, and Naruto, being the main character, is the one who gets it. It might have been crafted by a race of powerful beings in that other dimension, and it has powers that are not known in Naruto's own world.
The Raijin Sword in the fanfiction might have an ancient origin. It could be a weapon forged by a powerful ninja clan in the past. Maybe it was created to combat a great evil that once threatened the ninja world. And for some reason, it has now come into Naruto's possession.
There's also 'Naruto and the Legendary Blade'. This fanfiction has Naruto stumbling upon a legendary sword during one of his missions. The sword has a mind of its own and starts to guide Naruto in his training, leading him to become an even more formidable ninja. It's an exciting read as it shows Naruto's growth with the help of this unique weapon.
In the fanfiction, it might be that Naruto's poison blood is a result of the Nine - Tails' influence. The Nine - Tails is a powerful and malevolent force. Its chakra could have interacted with Naruto's blood over time, gradually turning it poisonous. This would also add an interesting layer to their relationship. Naruto would not only have to deal with the power of the Nine - Tails but also the consequences of its influence on his blood, which could change how he approaches training and his relationship with the other villagers who might fear his now - poisonous blood.
It could be an ancient heirloom from a long - lost wizarding family. Maybe it was passed down through generations and finally found its way to Harry.
One possible origin could be a long - lost kekkei genkai. Maybe Naruto's family had a secret bloodline ability that was suppressed for generations, and now it resurfaces as the red chakra.
Maybe it was some kind of experimental jutsu gone wrong or right. Some crazy scientist - type character in the Naruto universe might have been experimenting with different materials and ended up giving Naruto adamantium bones.
It could be due to a unique training method. In fanfiction, perhaps Naruto discovered a new way to purify his chakra through intense meditation or some sort of elemental training that no one else has tried before.
One possible origin could be a secret scroll he finds. Maybe it was hidden by an ancient shinobi who had the power to summon ravens, and Naruto stumbles upon it during one of his adventures.
In fanfiction, the chained blades could have various origins. One possibility is that they are a family heirloom that Naruto discovers. Maybe they were passed down from an unknown branch of his family. Another idea is that he finds them during a mission in some ancient ruins. The blades could be tied to a long - forgotten ninja clan that once had great power.
One possible origin could be a new type of training he undertakes. Maybe he discovers a hidden scroll that teaches him a unique form of physical conditioning that gives him super strength.