A new, powerful and mysterious dark wizard or witch could be the main antagonist. This character could have some hidden agenda that involves breaking Harry's support system. They could target his friends, use their weaknesses, and make them betray Harry for their own gain or to further the dark wizard's evil plans.
The Ministry of Magic could also be the main antagonist in this case. If the Ministry is corrupted or under the influence of dark forces, they could manipulate Harry's friends. They might convince his friends that Harry is a danger to the wizarding world order, and his friends, being naive or misinformed, end up betraying him.
In some fanfictions, Harry could be betrayed by friends due to jealousy. For example, if a friend is jealous of Harry's fame as the 'Chosen One'. Another reason might be a misunderstanding. Maybe Harry is wrongly suspected of using dark magic, and his friends, without proper investigation, turn against him.
The theme of redemption also frequently appears. Sometimes the betraying friend realizes their mistake and tries to make amends. In a particular fanfic, Hermione, who had initially betrayed Harry due to a mind - control spell, spends the rest of the story trying to undo the damage she caused and regain Harry's trust. It's an important theme as it shows that even in the face of such a huge betrayal, there can be a path to forgiveness and restoration of friendship.
It could be an outsider who manipulates Twilight's friends. This character might be envious of Twilight's popularity or power in the magical realm. They trick Twilight's friends into believing false things about her, making the friends turn against her.
It could be that an external force, like a dark wizard in disguise, is the main antagonist. This dark wizard targets Hermione because of her potential to be a great threat to their evil plans. They use magic to cloud the minds of her friends, making them believe Hermione is up to no good. The friends, being under this spell, betray Hermione without realizing they are being manipulated. The dark wizard then watches as Hermione struggles with this betrayal, all part of their grand scheme.
Well, usually in such fanfictions, Harry might face a lot of emotional turmoil and have to find his way alone to overcome the betrayal and the challenges that come with it.
In fanfiction, the 'betrayal by friends' trope often serves to create drama and character development. For Harry, it could be a shattering experience. However, it also opens the door for new alliances, like with Draco. Draco, who might have his own complex reasons, could see this as an opportunity to show a different side of himself to Harry. It could be a journey of redemption for Draco and a path of self - discovery for Harry.
It could be Draco Malfoy. Since Harry and Ron are not friends, Draco could take advantage of their weakened state. He might try to turn them against each other even more, perhaps by spreading false information about one to the other. He could also try to isolate Harry from the Gryffindor group since Ron is no longer on his side, making Harry an easier target for his bullying.
One interesting fanfic could be 'The Fallen Friendship'. In it, Harry discovers that his so - called friends have been secretly working against him due to a misunderstanding spread by a dark wizard. It shows Harry's journey of dealing with this betrayal and finding new allies.
In some fanfictions, it could be that Harry discovers a dark secret about the wizarding world that his friends don't believe or are too afraid to accept. His friends, thinking he has gone mad, side with the established order. They then betray Harry by reporting his so - called 'crazy ideas' to the Ministry of Magic, leading to Harry being hunted.
A common plot is the misinterpretation of Harry's actions. His friends might think he's being secretive about something dangerous, like a new form of magic he's discovered, when in fact he's just trying to protect them. Another plot could be that a new character enters the story and manipulates his friends against Harry for their own gain.