Maybe a new dark wizard or witch emerges. This character is extremely powerful and manipulative. They target Ron and Hermione because they are Harry's closest friends. By turning them against Harry, this antagonist hopes to isolate Harry and make him easier to defeat. The new antagonist could have some unique magical abilities that allow them to control the minds of others or create very convincing false scenarios.
There could be an ancient and evil magical creature. It has the ability to communicate with humans in a very subtle way. It plants seeds of doubt and distrust in Ron and Hermione's minds about Harry. This creature might have its own reasons for wanting chaos in the wizarding world, and by breaking up the trio, it starts a chain of events that could lead to destruction. It could be hidden in a very secret place in the wizarding world, only revealing itself when it needs to further manipulate the situation.
It could be that Ron's own insecurities and pride are personified as an antagonist. His jealousy towards Hermione's achievements and friendships could be like a dark cloud over him. Severus has to save Hermione from this negative side of Ron. So in this case, Ron's inner demons are the antagonists that Severus must combat.
Jealousy is a big one. Ron often feels like he's in Harry and Hermione's shadow. He might see their achievements and feel left out or unimportant, which could lead to his betrayal.
Hermione might seek revenge. Given her intelligence and magical abilities, she could use her skills to get back at Harry and Ron for their betrayal. This could lead to a big rift in their friendship and even endanger their common goals in the wizarding world. She might also side with other characters who are against Harry and Ron, changing the whole dynamic of the story.
Well, it could be that in these fanfictions, a dark influence overcomes Hermione and Ron. They might be manipulated by a powerful dark wizard who makes them believe that betraying Harry is the right thing to do. It could also be a miscommunication situation where they think they are helping but end up betraying Harry. For instance, they might be tricked into believing that Harry has turned bad and needs to be stopped.
One possible plotline could be that Ron and Hermione are influenced by dark magic or a powerful enemy who manipulates them into betraying Harry. For example, they might be under the Imperius Curse. Another could be that they misunderstand Harry's actions or intentions, leading them to believe he has gone to the dark side, so they turn against him in an attempt to 'save' the wizarding world. Maybe they are jealous of Harry's fame and power growing too much, and this envy festers until they decide to betray him.
It could be that a prophecy reveals something that makes Ron and Hermione think Harry is a danger to the wizarding world. They might decide to betray him in order to protect the greater good. However, as time goes on, they could realize their mistake. But by then, the damage to Harry's trust would already be done, and he would have a hard time forgiving them.
She might be heartbroken. Hermione is a strong - willed character, but being cheated on would surely shatter her. She could initially be in disbelief, given her trust in Ron. Then, she might become angry and start to distance herself from Ron, focusing more on her own studies or other relationships, like her friendship with Sirius.
Yes, there are some out there. One example might be on FanFiction.net. You can search for it using those keywords and look through the reviews to find a well - written one. However, different people have different ideas of what 'betrayal' means in this context, so you might need to read a few to find the one that suits your idea.
In some fanfictions, it might be because of a love triangle gone wrong. Suppose Ron had unrequited feelings for Hermione and thought she favored Harry too much. And Harry, perhaps misinterpreting Hermione's actions in a crucial situation, joined Ron in betraying her. This would be a complex and emotional reason for the betrayal.
Well, Harry might become very bitter. He could turn to other allies like Neville or Luna for support. He might also start to question all of his relationships and become more withdrawn.