In Harry Potter fanfiction, it could be an original character created by the author. Maybe a long - lost wizard who has been secretly training in the art of swordsmanship for years and emerges in the fan - created story.
Another possibility is that a character from a different magical realm in the fanfiction world could be the sword master. This character could bring a new style of swordsmanship and a different set of magical abilities related to the sword, adding an exciting new element to the Harry Potter - inspired story.
It could be a dark wizard who has found a way to control others from the shadows. Maybe someone who was once defeated but has now returned with this new and terrifying power.
It could be an unknown dark wizard who has been lurking in the shadows all this time. Maybe he was once a student at Hogwarts who was shunned and now wants revenge by controlling everyone.
Harry Potter himself could be a great sword fighter in fanfiction. He has shown great courage and quick reflexes throughout the series, which would translate well into sword fighting. He also has a connection to the Sword of Gryffindor, which could give him an edge.
Another possibility is a reformed dark wizard. Someone who has seen the error of their ways and now uses their in - depth knowledge of dark magic to teach students how to defend against it. This character would have a very interesting backstory, having once been on the wrong side but now using their experiences to help the next generation of wizards in the DADA class.
A new character created specifically for the fanfiction could also be the Master of Death. This character could be a hybrid of a wizard and a vampire or werewolf, having the combined knowledge and abilities of both worlds. They could use their power to try and create a new order where wizards, vampires, and werewolves co - exist peacefully, using their mastery over death to enforce certain rules.
It could be a former Auror who has turned to more shadowy ways of dealing with threats. After seeing the inefficiency of the Ministry's methods in some cases, they decide to take matters into their own hands and become a sort of assassin to target dark wizards.
The pure - blood supremacists could be the main antagonists. They might oppose Harry's ideas of a more inclusive and democratic political system as they believe in the superiority of pure - blood wizards.
It could be the Death Eaters who see Harry's new - found mastery of darkness as a threat to their own power and ideology. They might try to either recruit him or destroy him. Voldemort himself could be an antagonist, feeling that Harry is encroaching on his territory as the most powerful dark wizard.
Maybe it's an older wizard who has been studying arithmancy in secret for years. He decides to come out of the shadows and share his knowledge at Hogwarts, becoming the central figure in the story as he mentors the students and uses his skills to solve various magical problems.
A Muggle - born student could be the main character. They might not have the same preconceived notions about magical beasts as the pure - bloods and half - bloods. Their fresh perspective could lead to new ways of handling and understanding the beasts. They could be the one to discover a new breed of beast that has been hiding in plain sight, right on the outskirts of Hogwarts.