I'm not sure about its popularity. Fanfictions can vary greatly in how well - known they are. It depends on how many people are interested in the crossover concept of Naruto and Slender Man. There are so many fanfictions out there that it's hard to say if this one has a large following.
There's no real way to tell for sure if it's popular without looking at specific fanfiction platforms and their stats. Some people might find the idea of Naruto being the son of Slender Man really interesting and share it a lot, which would make it popular. But others might think it's too out - there and not pay much attention to it, so it could also be relatively unknown.
In this fanfiction, it might start with a strange backstory of how Naruto came to be the son of Slender Man. Maybe there was some sort of otherworldly interference. His relationships with his friends like Sasuke and Sakura would be very different. They might be scared or intrigued by his new heritage. And Naruto himself would have to deal with the fact that he's not what he thought he was, all while facing the usual challenges in the Naruto universe, like fighting enemies and protecting his village.
Yes, in some fanfictions, Harry is depicted as the son of Slender Man. Fanfiction often takes creative liberties and creates unique relationships between characters.
One popular theme is the mystery surrounding the Slender Man. Writers often explore his origin and what makes him so menacing. Another theme is the psychological horror of those who encounter him. For example, how their minds slowly break down under his influence. Also, the idea of survival in a world where the Slender Man exists is a common theme, like characters trying to find ways to escape his reach or protect their loved ones from him.
There are some original characters created by fanfiction writers that have gained popularity. For example, a young girl who has psychic abilities and can sense the Slender Man's presence from a distance. She becomes a key figure in trying to fight against him. These original characters often bring new perspectives and storylines to the fanfiction world of Slender Man.
One common plotline is Harry trying to hide his identity. Since Slender Man is often seen as a menacing figure, Harry doesn't want others to know his relation.
One popular story might be about Slender Man forming an unexpected bond with another male character in a spooky forest setting. It could explore their relationship as they face the mysteries of the forest together.
Slender Man has a huge fan base already. When those fans see the potential for crossovers, they get excited. Crossovers can also introduce new fans to the Slender Man concept. For instance, if someone who loves 'Attack on Titan' reads a crossover with Slender Man, they might become interested in the original Slender Man lore as well.
Maybe in the fanfiction, Slender Man has an alliance with Orochimaru. Orochimaru is always interested in strange and powerful beings. Together, they plan to take over the ninja world. Naruto and his friends then have to not only face Orochimaru's usual schemes but also deal with the terrifying abilities of Slender Man.
One popular Slender Man story is about his appearance in the woods. People claim to see him lurking among the trees, always just out of clear sight. His tall, slender figure is really creepy.
No, the Slender Man is not real. It was created as a fictional character in an online Photoshop contest. But the stories and the idea of him have had a real impact on people, especially in some disturbing cases where people were influenced by the concept and did bad things.