In the fanfiction, Arcturus Black is likely an important figure in the wizarding world. He might be a powerful and knowledgeable wizard from the Black family. Maybe he has a long - standing reputation for his magical abilities and wisdom, which is why he is chosen to train Harry Potter.
Arcturus Black might be portrayed as a wizard with a deep understanding of ancient magic in the fanfiction. He saves Harry Potter perhaps because he sees in Harry the potential to defeat the dark forces that are plaguing the wizarding world. His character could be a blend of mystery, power, and a sense of moral obligation towards Harry.
In such a fanfiction, Harry might grow up with a different set of values and skills. Arcturus Black could instill in him a more traditional pure - blood wizarding family's knowledge. He may have different magical training compared to what he got in the original series, perhaps more focused on family - specific magic or ancient spells.
His character could develop in a more sophisticated way. Arcturus may expose Harry to a world of high - society wizarding culture. Harry might become more refined in his speech and manners. Also, he could be more secretive and less trusting of others outside of Arcturus' influence, as he's being raised in a more exclusive environment.
One interesting plot point could be how Harry's upbringing by Arcturus Black changes his perception of the wizarding world. Maybe he's taught different magic than what he would have learned at the Dursleys or Hogwarts. Another could be how his relationships with other characters, like Sirius, are altered. Since Arcturus is an older Black family member, he might have different views on family loyalty compared to Sirius.
In some Harry Potter fanfiction, Arcturus Black might help by using his family's influence. For example, the Black family has a long history and certain political power within the wizarding world. He could use this to protect Harry Potter from some of the more dangerous pure - blood supremacist groups. He may also offer Harry guidance on the complex politics and social hierarchies of the wizarding world, drawing on his own experiences as a member of an old and influential family.
A major theme is the uncovering of secrets. In many of these fanfictions, Arcturus Black holds some sort of secret knowledge related to the wizarding world, perhaps something about the Dark Arts or the history of Hogwarts. Harry, being the curious and brave character he is, is drawn into this web of secrets. And also, friendship is a theme. Despite the age and family differences, a bond of friendship or a deeper connection forms between them.
One possibility could be that Arcturus Black imparts unique magical skills to Harry Potter. Maybe he teaches Harry some ancient and powerful spells that are not commonly known at Hogwarts. This could give Harry an edge in his battles against Voldemort and his Death Eaters.
One possible plot could be that Harry discovers a long - hidden connection between his family and Arcturus Black. Maybe Arcturus has some secret knowledge about Harry's parents or his destiny. They could team up to fight against a new dark force emerging in the wizarding world, with Arcturus using his old - fashioned but powerful magic and Harry his more modern skills learned at Hogwarts.