He might be a character who was involved in the politics of the wizarding world. Fanfiction could explore how his influence, either positive or negative, affected the balance of power. And this could in turn impact the events that Harry has to face. For example, if Arcturus Black had made certain alliances, it could change the dynamics of the war against Voldemort.
Cassandra Black can fit in many ways. She could be a key figure in uncovering forgotten magic or prophecies. For example, she might have knowledge of an ancient spell that could help Harry defeat Voldemort.
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.
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 fanfiction, umbramancy can fit in by being a long - lost or forgotten magic. It could be something that was once used by ancient wizards but lost over time. For example, it might be rediscovered in the Forbidden Forest by a curious student.
Nyarlathotep could be a new, powerful and malevolent force that disrupts the balance of the Harry Potter universe. He might be an entity from a different dimension that finds its way into the wizarding world, causing chaos.
Cortana might be seen as a powerful magical tool in the Harry Potter fanfiction world. Some might write that she has the power to enhance a wizard's magic. Imagine a wizard using Cortana to boost the power of their spells. She could also be used to store magical knowledge, like a magical hard drive, and wizards could access this knowledge to learn new spells or techniques.
Mithril might be incorporated into the fanfiction as a material that has a special connection to certain magical creatures. Maybe unicorns are attracted to Mithril, and a plot could revolve around a group of students trying to use Mithril to lure a unicorn to safety from a poacher. Along the way, they learn more about the magical properties of Mithril and how it interacts with the magic of the unicorn, which in turn could have implications for other areas of magic within the 'Harry Potter' universe.
Freerunning can fit into the Harry Potter universe by adding a new dimension to movement. For example, it can be used for escaping from dark wizards in the narrow alleys of Diagon Alley or Hogsmeade. The characters can use their parkour skills to quickly jump over obstacles and get to safety.
Illumi can fit in as an outsider with unique skills. His combat and stealth abilities are not like anything in the wizarding world, so he could be a wildcard. For instance, in some fanfictions, he might be seen as a new kind of threat or an unexpected ally to the Order of the Phoenix.