It could be a rogue Death Eater who has mastered a new form of invisibility spell. This Death Eater might be seeking revenge on those who opposed Voldemort.
It might be an ancient evil that has been awakened in the wizarding world. This entity has the power to be unseen and is drawn to the power of Hogwarts. It starts to target people randomly at first, but then seems to have a more sinister plan that involves Harry Potter. The assassin could be an otherworldly being with powers that are difficult to understand and combat.
Well, it might be about a mysterious assassin in the Harry Potter world who is unseen. Maybe this assassin targets important wizards and witches, and Harry Potter has to figure out who it is before more chaos ensues.
Perhaps an international wizard who has been trained in both magic and martial arts in some far - flung magical school. This wizard - assassin has his own code of ethics and when he meets Harry, he decides to take him under his wing, believing that Harry can be a force for good in a different way than the traditional Hogwarts - trained wizards.
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.
One possibility could be a rogue Death Eater. They have the skills and the inclination towards dark magic. For example, Bellatrix Lestrange could have easily fit the role of a dark assassin. She was extremely powerful and had no qualms about using dark magic to achieve her ends.
Maybe it's a character who has a hidden identity. It could be someone who was thought to be dead but actually survived and is now seeking revenge. This person could have a grudge against Moody for something that happened in the past. Or it could be a new character created specifically for the fanfic, a dark wizard from a far - off land who has been hired to kill Moody.
I don't know who the author of 'the world unseen novel' is. There may be many novels with that name or a similar title out there.
Harry Potter himself could be the main character in a 'harry potter ninja assassin fanfiction'. Imagine if he discovered a hidden part of his heritage that involved ninja - like training. He could then use his new skills along with his magic to fight against Voldemort and his Death Eaters in a more stealthy and unexpected way.
Yes, there are likely to be such fanfictions. The 'Harry Potter' universe has a vast and creative fanfiction community. Some fans might come up with the idea of Harry Potter being a Muggle assassin as a way to explore different storylines and character developments outside of the original canon. It could involve Harry using his magical skills in a Muggle - centered, assassin - like role, perhaps to protect Muggles from dark magic threats in a more covert way.
It could be a new character, like an assassin from a far - off land who comes to Hogwarts under false pretenses. They have their own set of skills and motives that are different from the regular characters in the Harry Potter series.
Well, one possible plot could be that Harry discovers a new threat that is specifically targeting Muggles. The wizarding world is either unable or unwilling to help, so Harry decides to take matters into his own hands. He uses his magic in secret ways to blend into Muggle society and eliminate the threat one by one. For example, he might use a modified version of the Disillusionment Charm to stay hidden while stalking his targets.