Power is a common theme. The oc often has to deal with Voldemort's thirst for power and either resist it or be drawn into it. For example, in many fics, the oc discovers a new form of magic that Voldemort wants to use for his own evil purposes.
It might be considered somewhat common. Since Voldemort is such an iconic and complex villain in the Harry Potter series, adding an oc to his fanfiction gives writers the chance to explore different relationships and storylines. For example, an oc could be a love interest for one of Voldemort's followers, or a secret weapon against him. However, it's not the most dominant trope, but still quite popular among those who write fanfiction in this genre.
It could mean that an original character (oc) is incorporated into a fanfiction story related to Voldemort. Maybe the author of the fanfic wanted to add a new element to the existing Voldemort - centered story by creating their own character and making it interact with Voldemort or the events in that wizarding world.
One popular voldemort oc fanfiction is 'The Dark Lord's Heir'. It has an interesting plot where the original character is intertwined with Voldemort's past and future plans in a very unique way.
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.