There's a fanfic called 'False Alliances'. After the war, Potter discovers that some of the people he worked with during the war had ulterior motives. They betray him to gain control over certain dark magic artifacts that Potter was protecting. It shows how the after - effects of war can create a climate of mistrust and betrayal.
One possible plotline could be that some of the former Death Eaters who were pardoned as part of the post - war reconciliation secretly plot against Harry. They might spread false rumors about him misusing his power or being a threat to the wizarding world again. Another could be that some of his own friends turn on him out of jealousy. Maybe they feel overshadowed by Harry's continued fame and hero status and start to believe the lies being spread about him.
Another instance could be Hermione betraying Harry in fanfiction. Suppose she falls in love with someone who is against Harry, like a pure - blood supremacist in a different version of the wizarding world. Her love for this person makes her go against Harry, which is a huge betrayal considering their close friendship in the original story.
It's popular because it adds a new layer of drama. The war is over, but the problems don't end. It shows that even in victory, there can be backstabbing. People like to explore the darker side of human nature in the context of the Potter universe.
I'm not sure of specific examples off the top of my head. But in general, in such fanfiction, it might involve Harry being betrayed by those in his so - called harem, perhaps by lies or broken trust. For instance, a character he trusted might turn out to be working for the dark side and use their relationship to get information on him.
I'm not sure specifically which 'Harry Potter betrayed' slash fanfictions exist. But in general, a 'betrayed' theme might involve a character like Harry being betrayed by a close friend, perhaps Ron or Hermione in some dark, alternate storyline created by the fanfic writer. It could explore themes of broken trust and how Harry copes with such a betrayal.
It could be Ron. Maybe he starts to feel that Harry has changed too much after the war and is no longer the friend he once knew. He might be influenced by outside forces or his own insecurities. Another possibility is Hermione. If she gets too involved in the new Ministry politics and starts to believe that Harry's actions are not in line with the new wizarding order, she could turn on him.
One good 'Betrayed Percy Jackson' fanfiction is 'The Fallen Hero'. In it, Percy is betrayed by his so - called friends at Camp Half - Blood who believe false rumors about him. Another is 'Alone and Betrayed' where Percy is framed for a crime he didn't commit and has to fend for himself. There's also 'Betrayal's Aftermath' which shows Percy's journey of rebuilding his life after the betrayal.
In some 'percy betrayed' fanfictions, Percy might be betrayed by his friends. For example, they could believe false rumors about him and turn their backs on him. This could lead to Percy feeling isolated and having to fend for himself, often resulting in him finding new allies or uncovering secrets that change the way he views his old friends.
One popular 'Harry Potter betrayed after 5th year' fanfiction could be 'The Fallen Hero'. In this story, Harry is betrayed by some of his closest friends due to a misinformation campaign by the Ministry of Magic. Another is 'Alone in the Dark' where after the 5th year, false accusations lead to his isolation and betrayal within Hogwarts. There's also 'Betrayed and Broken' which shows a very dark path for Harry after being betrayed, as he has to struggle not only with the external threats but also his own inner demons.
One possibility could be that the wizarding world turns on Harry after the war. Maybe they start to fear his power and influence. They could spread false rumors about him being a dictator in the making. His friends might be pressured to turn away from him, and he would be left alone, feeling betrayed by those he fought for.