Perhaps in this fanfiction, Naruto discovers some ancient magic similar to Voldemort's dark magic. He becomes intrigued by Voldemort's rise to power and starts to study his methods. However, Naruto's friends would be horrified as they see him drifting towards the dark side. Naruto might think he can use Voldemort - like tactics for good, but it's a slippery slope.
Another possibility is that the wizarding world in the Harry Potter side of the fanfiction might see Naruto as a new threat similar to Voldemort. So, some powerful wizards could come to the Naruto world to stop Naruto from becoming too much like Voldemort, thus making them the antagonists in this strange crossover fanfiction.
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.
In many fanfictions about Severus and Voldemort, their relationship is often complex. Severus, on one hand, was a double - agent. He was working for Dumbledore while seeming to be on Voldemort's side. This creates a lot of interesting dynamics in fanfiction. Writers might explore the power struggle between them, as well as Severus's internal conflict. Some fanfictions may also focus on how Voldemort views Severus, whether he truly trusts him or not.
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.
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.