In a 'Harry and Sirius join Voldemort' fanfiction, it might start with Voldemort making a very tempting offer. For Harry, it could be something related to saving his friends or family. Sirius might be lured by the idea of protecting Harry in a different way. However, once they join, they would face a moral dilemma. They would have to pretend to be on Voldemort's side while still holding onto their own values. And there would be a lot of internal conflict as they see the evil deeds happening around them.
Some might write it to explore a different side of the characters. It's a way to show how Harry and Sirius could be pushed to their limits and what they would do in extreme situations.
Well, if Sirius takes Harry to Voldemort in a fanfic, it would be a huge deviation from the normal story. One possibility is that Sirius is trying to make some sort of deal with Voldemort to save others. But Voldemort is not one to be trusted. Harry, being the Boy Who Lived, is a valuable asset to Voldemort for his own dark purposes. It could lead to a tense stand - off where Harry has to use his own wits and magic, even at such a young age, to try and escape the clutches of the Dark Lord.
Yes, there are some. You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Some stories might explore an alternate universe where Harry joins Voldemort for various reasons, such as being manipulated or having a different view of the wizarding world's power structure.
One possible plotline could be that Harry is tricked by Voldemort. Voldemort might use some powerful dark magic or false promises to make Harry believe that joining him is the right thing to do. For example, Voldemort could claim to have a way to end all the bloodshed in the wizarding world if Harry helps him.
Sirius could use his knowledge of the wizarding world's secrets. He might know some hidden places or spells that could keep Voldemort at bay. For example, he could put a powerful protective charm on the place where Harry is staying.
This is a very strange and dark concept for fanfiction. In a possible plot, perhaps they are under some powerful mind - controlling spell. Maybe Voldemort has found a way to use their skills and knowledge for his own evil ends. For example, Harry's connection to the Deathly Hallows could be exploited, and Ginny's powerful magic, which was shown in the books, could be used to carry out Voldemort's orders against their will.
Well, if Harry is captured by Voldemort in fanfiction, it could lead to some really dark and intense scenarios. Voldemort might torture Harry to get information about the resistance. Harry, on the other hand, could be secretly planning an escape while pretending to be broken. There could also be a rescue mission by his friends, which would be full of danger and drama.
In such a fanfiction, Harry might become a very different person. He could be more in tune with the Dark Arts. Maybe he would have a skewed sense of morality, influenced by Voldemort's views. He might also be more powerful in the Dark Arts but struggle with his identity as he is constantly in conflict between what he's been taught by Voldemort and his own inner goodness.
In such a fanfiction, Harry might be raised with a very different set of values. Voldemort could try to mold Harry into his own image, perhaps teaching him dark magic. But Harry might also secretly hold on to his own sense of good. For example, he could use the knowledge Voldemort gives him to ultimately turn against him.
One possible fanfiction could be about Sirius Black being in a situation where he has to face Voldemort alone. Maybe he discovers a secret weakness of Voldemort that no one else knows. Sirius, with his brave and reckless nature, decides to take advantage of this knowledge to try and take down the Dark Lord once and for all.