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.
A fanfiction could center around Sirius Black's attempts to rescue someone from Voldemort's clutches. He uses his skills as an Animagus and his knowledge of the wizarding world's hidden paths to get close to Voldemort's lair.
It's possible in fanfiction, but it would be a huge departure from his character. Sirius Black is a Gryffindor, known for their courage and loyalty to the side of good. He has personal reasons to hate Voldemort, like the death of his friends at the hands of the Death Eaters. But fanfiction authors might create a complex backstory to justify his joining. Maybe they could write that he was under a powerful spell or was tricked into believing Voldemort had different intentions. Still, it would shock most fans who know the original Sirius.
A popular plot device is the discovery of a 'hidden truth'. For example, Sirius might find out that there is some sort of family connection or prophecy that makes him think he should be on Voldemort's side. Another common one is the use of a love interest. Maybe a Death Eater or someone close to Voldemort seduces Sirius into joining. And sometimes, it could be a situation where Sirius is desperate and feels that Voldemort is the only way to achieve a certain goal, like saving someone he loves who is in danger from another source.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.
Yes, it is considered controversial. Harry Potter is traditionally seen as the hero who opposes Voldemort, the ultimate villain. The idea of him joining Voldemort goes against the core values of the original story. Fans of the original series often have strong emotional attachments to the characters as they are in the books, and this concept can be seen as a perversion of those characters.
Perhaps Harry is betrayed by the wizarding world as a whole. The Ministry of Magic wrongly accuses him of a serious crime. With no one to turn to, Voldemort approaches Harry. Voldemort shows Harry a world where he can be in control, instead of being at the mercy of those who have turned against him. So, Harry decides to join Voldemort out of desperation and a thirst for revenge against those who betrayed him.
Well, it could be that Evil Harry is actually a long - lost relative of Voldemort in this fanfiction. He discovers this connection and, feeling a sense of kinship with Voldemort's ideology of pure - blood supremacy, decides to join him. He then uses his skills at Hogwarts, like his knowledge of spells and potions, to enhance Voldemort's army. He might also be involved in turning some of his classmates to the dark side, creating a rift within Hogwarts between those who follow him and those who oppose Voldemort's new ally.
Perhaps Harry joins Voldemort out of a sense of hopelessness. After seeing so much destruction in the wizarding world, he might think that joining Voldemort is the only way to end the war quickly. However, he soon realizes that Voldemort's methods are too extreme and tries to find a way to turn things around from within the dark side.
One possible plot could be that Harry is influenced by Voldemort's false promises. Maybe Voldemort offers to bring back Harry's parents in exchange for his loyalty. Harry, in a moment of weakness and longing, decides to join him. However, once he realizes Voldemort's true nature of deception and destruction, he has to find a way to break free from this dark alliance.