Voldemort often appears as a sort of 'mentor' figure in some dark path fanfictions. His ideology and power attract characters to the dark side.
Of course, Draco Malfoy is also common. Given his family's history with the dark side, it's easy to imagine scenarios where he fully embraces the dark path in fanfiction and drags other characters with him into a world of dark magic.
Most likely, Harry Potter would still be one of the main characters. Since it's based on the Harry Potter universe, his actions and decisions would probably be central to the story even on this'rejected path'.
Voldemort is quite common. Since he's the main villain in the original series, fanfic writers often use him to create darker storylines. Bellatrix Lestrange is also popular. Her insane and violent nature gives a lot of room for dark interpretations.
Bellatrix Lestrange can be a significant character in these fanfictions. Since Harry is on a darker path, she might see him as a potential recruit or someone she can manipulate. Sirius Black may also be present, but his role could be one of trying to save Harry from his dark descent, which adds a lot of conflict to the story.
The characters in the 'dark trio' in the fanfiction are likely to be those who have a propensity for the dark arts or have some sort of malicious intent. It could be that the author takes minor characters from the original series and gives them more prominent roles within this dark context. For example, Crabbe and Goyle could be developed further into more menacing characters, or perhaps some of the Death Eaters that were not as well - explored in the original books could be part of this trio.
Obviously, Harry himself is the central character. Hermione and Ron are also often present. In some stories, they try to save Harry from the darkness, while in others, they become his enemies as he turns dark. Dumbledore may also play a significant role, either trying to guide Harry back to the light or being unable to stop his descent into darkness.
Another plotline could involve a group of students forming a secret society dedicated to exploring the dark path. They might start with small acts of mischief using dark magic but then spiral out of control as they delve deeper into the more dangerous aspects of the dark arts.
Well, obviously Harry himself is the central character. But aside from his friends, characters like Draco Malfoy could have a different relationship with Harry. Instead of being just a rival, he could be an ally of sorts in Harry's dark journey, or perhaps he tries to use Harry's newfound darkness for his own gain. Snape too could be important. Given his complex relationship with Harry, he might be in a position where he has to choose between helping Harry return to the light or letting him continue down the dark path.
In some cases, corrupt Ministry of Magic officials can be antagonists in 'Harry Potter Dark Business Fanfiction'. They could be involved in covering up or even profiting from the dark business. For example, they might turn a blind eye to the illegal trading of dangerous magical creatures in exchange for bribes. This would make them enemies of Harry Potter and those who strive for a just and clean wizarding world.
It's hard to say for sure without reading it, but it could be Harry Potter himself. Since it's related to the Harry Potter world, he might be the main character exploring the Library of Heaven's Path.
I'm not sure specifically as there are many different 'Path of Conquest' fanfics out there. But it could include the original characters like Harry, Hermione, and Ron, who might be on a new kind of adventure. Or it could introduce new characters who are the main drivers of the conquest story.