Harry Potter himself would likely be a main character. He's the central figure in the Potter universe. And maybe Darth Vader or another Sith Lord could be a major part of it too. As for the harem, Hermione and Ginny from the Potter side could be part of it, along with some female Sith characters, like Asajj Ventress if we're including Star Wars elements.
One interesting idea could be the clash of different magical and dark side elements. The Harry Potter universe has its own magic system, and the Sith from Star Wars bring a different kind of dark power. In a harem aspect, perhaps different female characters from both universes are brought together, each with their own skills and personalities, creating unique interactions and storylines.
If Harry Potter becomes a Sith in fanfiction, it would be a wild departure from his original character. He'd likely trade his Gryffindor values for the dark and power - hungry ways of the Sith. His magic could take on a more menacing form, and his relationships would be severely strained. For example, his friendship with Ron and Hermione might turn into enmity as he delved deeper into the dark side.
I'm not entirely sure as it's a very specific and perhaps not a mainstream concept. But it might be a fan - created story where the Harry Potter universe is combined with elements of Sith (from Star Wars) and the idea of pureblood. It could be about a Harry Potter - like character who has Sith - like powers and is of pureblood heritage in this new, mashed - up fictional world.
One idea could be a story where a Sith Assassin somehow gets transported to the Harry Potter universe. Maybe they initially see it as a strange and weak world compared to their own, but then they discover the unique magic there. They could try to use their Sith powers to dominate, but find that the magic of Hogwarts and its defenders are not so easy to overcome.
One idea could be a crossover where a powerful Sith Sorcerer somehow finds his way into the Harry Potter universe. Maybe he's on a quest for a unique magical artifact that only exists in the wizarding world. He could start causing chaos, and the Hogwarts students have to figure out a way to stop him using both their magic and their knowledge of the wizarding world's secrets.
A common theme is the struggle between good and evil. Since Harry is usually the main character who gets involved with Sith - like elements, it shows his internal battle between his original good - natured self and the dark influence. For example, in many of these fanfics, he might struggle with using dark magic for what he believes are good reasons.
I don't think it's a popular type of fanfiction. While there are many creative crossovers in the fanfiction world, the idea of making Harry Potter a sith pureblood is a rather strange and complex combination. It requires a deep knowledge of both the Harry Potter and Star Wars universes, and not many fans may be interested in such a convoluted mash - up. So it's likely not very popular.
Well, first, Harry would need to study the ways of the Sith. He has to understand his enemy. Then, he could combine his wizarding skills with some new knowledge. For example, he might learn how to use his magic to counter the Sith's mind - control abilities. He could also use his Patronus, which represents hope and light, to drive away the darkness of the Sith. And of course, he'd need to find the Sith's weakness, like in the way he found Voldemort's Horcruxes.
A new character could be introduced as a Sith Lord. Maybe a wizard who has been studying dark and forbidden magic from other dimensions. He could have a red lightsaber - like wand and use powers that combine magic and Sith - like abilities. For example, he could use mind control not just on muggles but also on some weak - willed wizards.