A powerful and ancient Pokémon from a long - forgotten era could be the cause. This Pokémon might have a grudge against Arceus for some past event and has finally found an opportunity to strike and hurt it. For instance, if Arceus had banished this Pokémon in the past, it could now be seeking revenge.
Most likely Arceus is one of the main characters. There could also be other Pokemon or mythical creatures in the story that interact with Arceus in the context of love.
There might be some new characters introduced in the fanfiction. A mysterious Pokemon trainer who has some ancient knowledge about Arceus and can guide Ash. Or perhaps a group of Pokemon researchers who have been studying the relationship between Arceus and the world for a long time. These characters could add more depth to the story, providing different perspectives on the situation between Ash and Arceus, and helping Ash in his fight against the perceived betrayal.
Arceus is a god - like Pokémon, so it seems likely that it would win. But in fanfiction, the rules can be bent. Ash has a lot of experience and a strong will. If he manages to form a powerful alliance with other legendary Pokémon, he might stand a chance against Arceus. However, it's still a long shot as Arceus has near - omnipotent powers.
Maybe a group of dark - type Pokémon that have banded together and are trying to overthrow Arceus. They view Naruto as an easy target at first, but quickly realize he is a formidable opponent with his ninja skills and new Pokémon - related powers. They would constantly try to undermine Naruto and his efforts to protect the Pokémon world.
Fanfiction can be created by a wide range of people. Anyone with an imagination and a passion for a particular fictional universe can take a stab at it.
There could be many different authors who created fanfictions with this concept. It's hard to pinpoint just one. Fanfiction is a vast world, and often these unique ideas are created by individual fans who are inspired by the Sonic franchise and want to put their own spin on it.
Definitely Pikachu. Pikachu is one of the most recognizable Pokémon, and having it interact with Arceus in a crossover would be really cool. It could be like a small, cute Pikachu following around the all - powerful Arceus.