In the 'Harry Potter Hides He Is Dark' fanfic, Harry could be hiding his dark side by carefully choosing his words. He wouldn't let slip any thoughts or opinions that might hint at his darker tendencies. He might also avoid certain places or people that could trigger his dark side, like staying away from dark magic artifacts or those who practice dark magic openly. And, of course, he could be using Occlumency not just to keep his thoughts private from Voldemort but also to hide his own dark nature from others.
The plot could develop with Harry secretly studying dark magic texts in the restricted section of the library. He might start to have strange dreams or experiences related to the dark magic he's exploring, and he has to be very careful not to get caught by Filch or the teachers.
He might hide it because he doesn't fully understand his own fascination. Harry has always been on the side of good, but perhaps there are elements of the dark side that seem alluring in a strange way. He could be afraid that exploring these feelings would lead him down a dangerous path, so he keeps it a secret.
The revelation of Harry being 'dark' would cause chaos in the wizarding world. People who once idolized him would feel betrayed. The Order of the Phoenix might question their support for him. Enemies could use this as an opportunity to turn the public against Harry. His relationship with Ginny would also be severely strained. She fell in love with the 'good' Harry, and this new side of him could be too much for her to handle. Also, it could potentially put Neville or Luna in danger if they continued to support Harry, as they might be seen as associated with his 'darkness'.
Well, it could be that the dark side has certain powers or artifacts that end up being the key to saving Harry in fanfiction. Maybe a dark wizard has a magical object that, when used in the right way, can reverse a curse on Harry or help him defeat Voldemort. The dark side's resources, which are often feared and shunned in the normal Harry Potter lore, can be turned into something positive for Harry's salvation in fan - made stories.
In some fanfictions, Harry might use powerful ancient magic that he discovers in a hidden library or from an old wizard. This magic could be specifically designed to counter the dark magic of the Dark Mark.
In some fanfiction, the dark side might help Harry by giving him unique powers. For example, dark magic could offer him abilities that are not available in regular magic, which he can then use to fight against stronger enemies.
Well, 'His Dark Lady' in a Harry Potter fanfic might be a character that the author has invented to add a new twist to the story. It could be related to the existing themes of good and evil in the Potterverse. For example, she could be a love interest for one of the main characters, but with a dark side that causes conflict. This could lead to all sorts of interesting plot developments, like moral dilemmas for the other characters or changes in the power dynamics within Hogwarts.
It usually happens through a series of traumatic events and powerful influences that twist his morals and goals.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.
He might use his clothing to conceal it. For example, if he has a cut on his arm, he could wear long sleeves or a cloak to cover it up.