Well, perhaps Asta could be corrupted by a powerful dark magic artifact. He might start to see his friends as obstacles in his quest for power and turn against them.
It could be Yuno. Normally they are friends, but if Asta turns evil, Yuno might be the one to oppose him. Yuno has his own strong magic and a sense of justice that would make him a natural adversary.
One possible plot could be Pikachu getting influenced by a dark and powerful Pokemon. For example, a Darkrai might use its nightmare - inducing powers on Pikachu. This could make Pikachu start to attack other Pokemon and trainers without mercy. Maybe Pikachu would first target its own trainer, Ash, as it would be the most shocking turn of events. This would create a lot of chaos in the Pokemon world and force Ash and his other Pokemon friends to try and figure out how to save Pikachu from this evil state.
Well, it could be that Chichi is betrayed by her friends and that turns her evil. Maybe she discovers some dark secrets about her past that make her lose faith in good. Or perhaps she is influenced by an evil force that corrupts her pure heart.
One possibility could be that they turn evil due to a powerful enemy using some dark magic or jutsu on them. This could make them lose their sense of self and start causing chaos in the Hidden Leaf. For example, they might start attacking their former friends and allies, believing they are now enemies.
One way Asta could turn evil in a fanfiction is if he was constantly ostracized for his lack of magic in a more extreme way. For example, if the Magic Knights not only rejected him but actively bullied him. This could lead to a build - up of anger and resentment. Eventually, he might decide that the system that has shunned him is wrong and needs to be overthrown, leading him down an 'evil' path of using his anti - magic to cause chaos rather than protect.
One possible plot could be Ruby being betrayed by her closest friends. Maybe she discovers a dark secret about the Huntsman academies that makes her question everything she has ever believed in. This disillusionment could start her on the path to evil as she feels that the system she has dedicated her life to is corrupt.
Another plot could be that he is exposed to some sort of mind - controlling substance or technology. The enemy could be using it to turn him against Gotham. Under this influence, he starts to see his friends as foes. He could begin to attack the Bat - family and other superhero allies. His actions would be erratic and violent, and the city would be in chaos as they try to figure out what happened to their hero and how to stop him.
A potential plot could be Asta's slow descent into darkness. He starts off as a bit more cynical after facing continuous rejections. Then, he meets a group of outcasts who have also been wronged by the system. They convince him that the only way to make things right is to take power by force. Asta begins to use his anti - magic to steal magic from other mages, believing it's a way to level the playing field. As he does this, he becomes more and more feared and is seen as a threat to the kingdom.
Perhaps in the fanfic, there is a misunderstanding. Asta might accidentally say something that makes Noelle think he doesn't care about her. This could lead to Noelle crying, and Asta then has to figure out what he said wrong and make it up to her. It could be a story about miscommunication and making amends.
Well, one possibility is that Ruby could be influenced by a powerful and evil relic. Maybe she discovers a dark side of the relic's power and becomes corrupted by it, gradually turning evil as she starts to value power over her friends and the greater good.