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.
In many fanfictions, Bellatrix is often depicted as extremely loyal to Voldemort. She is seen as his most devoted follower, willing to do anything he asks without question.
As a fan of online literature, I can't provide any bg related to Voldemort because Voldemort is a fictional character that doesn't exist in any real story or novel.
Maybe it's a completely new character created for the fanfiction. This new character could be from a far - off land, with a different kind of magic that Voldemort is interested in. Their marriage could be a way for Voldemort to gain access to that new magic or territory.
Another option could be a Muggle - born witch. Voldemort might have been intrigued by her in a different way compared to his hatred for most Muggle - borns. But as his views on pure - blood supremacy become more extreme, she can't bear to be with him any longer, and their relationship ends.
Harry often becomes more conflicted. He has always seen Voldemort as the ultimate evil, but now being mates, he has to re - evaluate his views. He might start to see the humanity in Voldemort, which makes him question his own beliefs and actions in the fight against the Dark Lord. For example, he may hesitate in battles more than before.
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.
Often in fanfiction, their relationship can change drastically. They could end up as allies, which would require both of them to change. Harry would have to see past Voldemort's evil deeds, and Voldemort would have to let go of his hatred for Harry. This exploration of character change makes fanfiction so interesting.
One possible reason could be that Snape fails to deliver crucial information. Voldemort is a very demanding and unforgiving master. If Snape doesn't give him accurate details about the Order of the Phoenix's plans, for example, Voldemort would surely be enraged and punish him.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.