Maybe an alien race that wants to conquer Casshan's world. They figure out how to brainwash Casshan to use him as a weapon against his own kind. This alien race would be the antagonists as they are causing the main conflict by brainwashing Casshan.
It could be a powerful AI or a mad scientist. They might see Casshan's power and want to control him by brainwashing, so they would be the antagonists.
One possible plot could be that an evil force brainwashes Casshan. They manipulate his memories and values. As a result, Casshan starts to target the people he used to protect. His friends then have to find a way to break the brainwashing, which could involve a long and difficult journey filled with many challenges and revelations about Casshan's true nature.
It's possible. First, consider what kind of 'brainwashed' aspect you're interested in. Is it a psychological exploration or part of an action - packed plot? Once you have a clearer idea, it'll be easier to search for. Also, reaching out to other Casshan fans in forums or groups might lead you to a great 'brainwashed casshan fanfic' as they may have come across one already.
I'm not entirely sure specifically what a 'casshan brainwashed fanfic' is without more context. It could be a fan - created story related to something named Casshan where a character is brainwashed. Maybe it's in the context of a particular anime or game.
One common theme could be the struggle against the brainwashing. Casshan might be fighting to regain his true self and free his mind from the control that has been imposed on him.
To find 'casshan brainwashed fanfic', begin with the major fanfic hosting sites. Archive of Our Own is known for its extensive collection of various fanfics. Type in 'casshan brainwashed' in the search function. FanFiction.net is also a possibility. Additionally, if there are Casshan - specific fan groups on platforms like Facebook or Reddit, it's worth checking there. Fans in those groups might be able to point you in the right direction or might have even written the fanfic you're looking for themselves.
It could be a rival from X Sans' past. Someone who has always been jealous of X Sans' power or popularity in the fanfic world. This character might have finally found a way to get revenge by brainwashing him.
The most obvious antagonist would be the Millennium Earl. He has the motive and the means to brainwash Allen. He wants to destroy the Black Order and Allen, with his Innocence, is a major threat to the Earl's plans. So, by brainwashing him, he can turn that threat into an advantage.
Another possibility is that the wizarding legal system or a corrupt official within it could be the main antagonist. They might be trying to make things difficult for Harry, or perhaps they are in cahoots with the Weasleys and are trying to cover up the Weasleys' misdeeds. This would add an interesting layer of complexity to the fanfic.
It could be the person who spreads false rumors about Jiraiya to Naruto. This person might be an enemy of Jiraiya who wants to drive a wedge between him and Naruto. By making Naruto ignore Jiraiya, they hope to weaken both of them.
Naruto is a likely candidate. Given his importance in the series, a plot where he is brainwashed can create a huge impact. His powerful chakra and his role as the main character mean that if he is brainwashed, it can change the entire dynamic of the story. The enemy could use his strength against his own friends and village.