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.
Well, it could be that Ash makes a discovery that goes against Arceus' grand plan and is thus betrayed. Maybe he finds out something about the balance of the Pokemon world that Arceus doesn't want known.
In a fanfic where Ash betrays and captures Arceus, it could be a very dark and unexpected turn. Maybe Ash has been influenced by some powerful and malevolent force. This would completely change his character archetype from the heroic trainer we know. It could lead to a lot of chaos in the Pokémon world as Arceus is like a god in that universe.
Team Rocket might be main characters as well. They could be constantly trying to capture Ash or his Pokemon because of his special status. Their bumbling attempts to take advantage of Ash's link to Arceus could add a lot of comical and action - filled moments to the fanfiction. And of course, Arceus itself could be a major character, guiding Ash in some way or interacting with him directly.
Obviously, Ash and Arceus are the main characters. Ash is the one being raised, and Arceus is the one doing the raising. But there could also be other Pokémon that are important in Ash's upbringing, like the Pokémon that Arceus assigns to look after Ash or be his companions during his growth.
I'm not sure exactly how many fanfictions exist where Ash is the son of Arceus, but it's a very interesting concept. It could lead to all kinds of plot twists. For example, Ash might have some hidden powers or a special connection to the Pokémon world that others don't. This kind of fanfiction can explore the relationship between Ash and Arceus in ways that the original series doesn't.
In 'ash raised by arceus fanfiction', one could expect a lot of creative elements. For example, Arceus being a god - like Pokémon might raise Ash in a way that completely changes his understanding of the Pokémon world. It could involve Ash learning secrets about Pokémon that no one else knows, and this might lead to new adventures and challenges for him.
Ash would likely be extremely heartbroken. He has always had a deep connection with Pokemon and believing that Arceus, a powerful and supposedly benevolent being, has betrayed him would be a huge blow. He might become withdrawn at first, not knowing who to trust anymore.
Perhaps Ash has been deceived by a false prophecy or a mysterious figure. This false information could lead him to believe that capturing Arceus is the only way to save the Pokémon world from an impending doom. So, in his misguided attempt, he betrays Arceus.
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.
It could be that Arceus betrays Ash by revealing that all of Ash's Pokemon encounters and relationships were fabricated. Arceus made it so that Ash would form bonds with certain Pokemon to further its own ends. For instance, Arceus wanted to study the power of friendship among humans and Pokemon through Ash's experiences. When Ash finds out, he feels used and betrayed. He might then set out on a journey to free his Pokemon from Arceus' control and restore their true free will.