I'm not entirely sure. While there is a large amount of Harry Potter fanfiction out there, the specific 'harry is evil and mated' concept might be a bit more niche. It could be that it exists on some smaller fanfiction platforms or within certain groups of fans who like to explore darker and more unconventional storylines.
One common theme could be the exploration of power. If Harry is evil, his use and abuse of magic power in a different way than in the original story. And the'mated' aspect might bring in themes of loyalty and possessiveness in relationships that are twisted due to his evil nature.
The main characters are clearly Harry and Severus. Harry, in this fanfiction, is depicted as evil, which is a big departure from his original character in the canon. And Severus, who has his own complex personality, is mated to Harry.
One popular 'evil harry is mated to tom' fanfiction is 'Dark Alliances'. It explores the complex relationship between an evil - leaning Harry and Tom in a really interesting way, with a lot of plot twists.
One possible interesting plot could be the conflict it creates within the wizarding world. If Harry is evil and mated to Severus, it would shock everyone. Their relationship would likely be a source of great turmoil, with friends and foes alike trying to figure out how to deal with this unexpected union.
The character development in these fanfictions is a big part. Harry's transformation into an 'evil' version and how Tom responds to this new Harry. Their relationship also affects other characters around them, like their friends and enemies. There could be a lot of political intrigue in the wizarding world due to their mated status, as different factions might try to use or oppose them.
It varies from one fanfiction to another. Some might depict it as a very magical and ritual - based process, perhaps involving special spells or potions unique to the fairy kind.
Harry is surely a main character. As for the mutant, it could be a mutant with powers like telepathy or super strength. If it's a crossover fanfiction, it might be a popular mutant like Wolverine or Jean Grey. However, in an original take, the mutant could be a completely new character with unique powers and a backstory that ties in with Harry's story in interesting ways.
A vampire hunter could be a key character. They might initially see Harry as a target too because of his association with the vampire. This would create a lot of tension as Harry and his mate have to either convince the hunter that they are not a threat or fight against them. And don't forget about Dumbledore (assuming this is in the Harry Potter universe). His reaction to Harry's new situation would be very interesting to explore.
Perhaps a key plot point is the challenges they face together. For example, they might have to deal with enemies who are against their relationship. Or it could be about how they combine their powers, Harry's magic and the mutant's abilities, to overcome difficulties. Also, the discovery of their mating bond in the first place could be full of mystery and drama.