The teachers at Hogwarts, like Dumbledore (if he were still alive in the fanfic), would be deeply concerned. Dumbledore always saw great potential in Harry for good. He would probably try to understand what led Harry to this path and would use all his wisdom and magic to try and turn Harry back. The other teachers would also be on high alert, fearing for the safety of the students and the future of the wizarding world.
In such a fanfic, perhaps Harry Potter is magically manipulated. The Dark Lord could use some powerful dark magic to cloud Harry's judgment. Harry might then start to use his skills and knowledge, which were originally meant for good, to help the Dark Lord's evil plans. For example, he could use his Parseltongue ability to communicate with dark creatures for the Dark Lord's purposes.
Hermione would be devastated. She has always been Harry's loyal friend and believed in the fight against the Dark Lord. She would probably try to convince Harry to come back to the right side, reasoning with him about the evil nature of the Dark Lord's actions. Ron, on the other hand, would be both angry and confused. He might initially think it's some sort of joke or that Harry has been tricked.
I really like 'The Dark Lord's Son: A New Path for Harry'. In this fanfic, Harry is raised by the Dark Lord's followers after Voldemort's victory. It shows a completely different Hogwarts, full of dark magic teachings. The story is full of twists and turns as Harry starts to question the world he's living in.
It could be that the Dark Lord offers Harry something irresistible. Like a way to bring back his parents. Harry, being desperate to have his family back, might be tempted to join the Dark Lord in the fanfiction. After all, the love for his parents was always a strong driving force in his character, and this could be a major turning point that leads him to side with the Dark side.
Well, in 'Dark Lord Harry Potter fanfic', the most obvious change is Harry's alignment. Instead of being the hero, he becomes the villain or at least an anti - hero. This completely flips the moral compass of the story.
Another good one could be 'Dark Lord Potter and the Hogwarts of Shadows'. In this fanfic, the dark lord Harry Potter has a different set of relationships with the other characters. The author does an excellent job of building a new Hogwarts environment that's filled with mystery and danger. It really explores the idea of what would happen if Harry took a darker path in a detailed way.
One popular 'Harry Potter' fanfic about the next Dark Lord could be 'The Heir of Darkness'. It presents a really interesting scenario where a new dark force rises in the wizarding world long after Voldemort's fall. The character development is quite good, especially of the new 'Dark Lord' - who has motives that are a mix of personal vendetta and a twisted view of the wizarding hierarchy.
Well, there are fanfics where the Dark Lord actually has a change of heart in the 'Harry Potter Dark Lord fanfic' genre. It might start with him seeing the error of his ways after experiencing a vision of a desolate future caused by his actions. Some fanfics also explore his relationship with his Death Eaters in a more complex way, showing that not all of them are blindly loyal and there are power struggles within the dark ranks.
The concept of Harry as a Dark Lord in fanfiction is alluring because it subverts the original story. It allows fans to explore the 'what if' scenarios. What if Harry had given in to the power-hungry side? It gives a chance to rewrite the power structure in the wizarding world.