In some fanfictions, Voldemort wins through a change in the prophecy's interpretation. If the prophecy was misread, it could give him an edge he didn't originally have.
He might win by outsmarting Dumbledore. If Dumbledore makes a miscalculation or fails to anticipate one of Voldemort's moves, it could lead to Voldemort's victory. For example, if Dumbledore is distracted by protecting something else and Voldemort seizes the opportunity to attack at a crucial moment.
Another way could be through internal betrayal within the Order of the Phoenix. If someone in the Order turns on their comrades and gives Voldemort valuable information or helps him directly, it could tip the scales in his favor.
In voldemort wins fanfiction, there might be a lot of character development that goes against the original. For instance, some of the Death Eaters could become more complex characters. They may start to question Voldemort's extreme rule once they see the destruction it causes. Also, there could be a different take on the prophecy and how it was misinterpreted which led to Voldemort's win.
Bellatrix Lestrange would probably be a main character. As one of Voldemort's most loyal followers, she'd be at the forefront of his new regime. She might be in charge of torturing those who oppose Voldemort.
Well, in such fanfiction, Voldemort's possessiveness might show through his constant attempts to control Harry's actions. He may try to isolate Harry from his friends and allies, wanting Harry all to himself in a very dark and twisted way. For example, he could use dark magic to keep Harry in a certain place or manipulate situations so that Harry has no choice but to be near him.
Maybe Voldemort manages to defeat all the major resistance. He could start by taking out key members of the Order of Phoenix one by one. For example, he might ambush Sirius Black early on, weakening the Order's intelligence network. Then, he'd target Dumbledore, perhaps using some dark magic that even Dumbledore couldn't counter. With the two biggest threats out of the way, he'd be able to overrun the wizarding world, starting with Hogwarts. He'd install his own regime, making pure - blood supremacy the law and punishing those who oppose him severely.
Often, Voldemort starts as this purely evil figure. But when he adopts Harry, he begins to show some unexpected softness. For example, he might start to protect Harry from other threats, which is very different from his usual self.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Well, in some fanfictions, Voldemort might see Harry as a tool to be used against his abusers. He could use this knowledge to manipulate Harry into joining his side, thinking that he can offer protection or revenge against those who have wronged Harry.
In this fanfiction, Voldemort's character is explored in a way that challenges our traditional view of him. His seeing Harry abused might make him reevaluate his relationship with Harry. He may start to see Harry not just as the Boy Who Lived who he wants to defeat, but as a person who has suffered, much like he might have in his own way. Harry, on the other hand, is stripped of some of his heroic facade. His abuse makes him more relatable in a sense. We can see his pain and how it affects his actions and decisions, and also how he reacts to Voldemort's new - found interest in his situation.
Maybe Harry uses some powerful and unexpected spells. For example, he could combine the Patronus Charm with a blasting spell. The Patronus might distract Voldemort while the blasting spell hits him hard.
It might start with Voldemort's initial curiosity. He realizes Harry is more than just a threat. So he begins to observe Harry closely. As time goes by, Voldemort may start to feel a strange sense of connection, like he's the only one who truly understands Harry's power.