Yes, it is somewhat popular. There are many fans who like to explore alternative storylines where Harry has a secret connection to the Dark Lord. It adds an interesting twist to the original story and allows for unique character development.
The contrast. Harry is known as the hero, so making him a secret Dark Lord fanfiction is a huge contrast to his normal character. It creates a sense of mystery.
The concept of Harry as a Dark Lord in fanfiction is alluring because it subverts the original story. It allows fans to explore the 'what if' scenarios. What if Harry had given in to the power-hungry side? It gives a chance to rewrite the power structure in the wizarding world.
One interesting prank could be to magically change the Dark Lord's robes into something ridiculous like a frilly pink dress. It would be a shock to his dignity and his followers' perception of him.
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.
One possible take on a 'Dark Lord Harry Potter' fanfiction could be a story where Harry, instead of following the path of a hero, is influenced by dark forces early on. Maybe he discovers a hidden, dark side of magic that the Hogwarts faculty has been suppressing. This could lead to a completely different power dynamic within the wizarding world.
In many 'Harry Potter' fanfictions about 'Dark Lord Rising', it often explores the resurgence of Voldemort in new and creative ways. It might show different scenarios of how he regains power, like through dark magic artifacts that were not fully explored in the original books.
It might not be as mainstream as some other genres. Most fans prefer to see Harry as a hero rather than a Dark Lord supporter. However, in the vast world of fanfiction, there are always niche ideas, and this could appeal to a small subset of fans who like exploring darker and more subversive storylines.
Power is a common theme. Fanfictions often explore how the Dark Lord amasses and wields his power. Another theme is fear, as his presence instills fear in the wizarding world. Redemption can also be a theme in some, where the Dark Lord or his followers might have a chance at redemption.
Severus Snape's memories or a ghostly apparition of him could be a main character. Since he had such a complex relationship with the dark lord, he could offer insights to the daughter. Also, Ginny Weasley could be significant. She's a strong witch in her own right and could be a role model for the dark lord's daughter. And then there are the professors at Hogwarts like McGonagall who would have to deal with the situation of having the dark lord's daughter at school.