It could be an ancient heirloom from a long - lost wizarding family. Maybe it was passed down through generations and finally found its way to Harry.
It's possible that the sword has its origins in a different magical realm. In the fanfiction, there could be a connection between Harry's world and another dimension, and the sword was brought over from there. It could have been a weapon used by great heroes in that realm and now it has come to aid Harry in his own battles.
The sword might be a gift from a mysterious magical being. This being saw potential in Harry Potter and decided to give him the sword as a tool to help him in his future battles. It could have been enchanted specifically for Harry to face the unique challenges he will encounter.
In some fanfictions, it could be a gift from a parallel magical dimension. There could be a race in this other dimension that is very advanced in combining magic and metalwork, and they see Harry as a hero worthy of such a gift. The metal arm could then hold the secrets and powers of that dimension, which Harry has to figure out how to use to save the wizarding world.
One possible scenario is that Harry discovers the Force during his time at Hogwarts. Maybe there's a hidden chamber or an old artifact that unlocks this power within him. Another scenario could be that he meets a character from a different universe who teaches him about the Force. For example, a rogue Jedi who has somehow ended up in the wizarding world.
One possible magical property is that it can transform. It could turn into a different object when needed, such as a sword for combat or a shield for protection. It might also have the ability to store spells. Harry could load up the cane with spells in advance and then release them at a later time when he's in a difficult situation.
One possible consequence could be his expulsion from Hogwarts. Using black magic is strictly prohibited in the wizarding world, and the school has a zero - tolerance policy. Another consequence might be the loss of his friends. His friends, who uphold the values of good magic, might not be able to accept his use of black magic.
One possible plotline could be an alternate universe where Harry is turned to the dark side early on. Maybe he discovers some false information about his parents' death and blames the wizarding world, leading him to use Avada Kedavra on those he believes are responsible.
Well, it could potentially give him an edge in combat. Maybe he can use it to defeat dark creatures more easily. For example, if he faces a Dementor, the sword might be able to disrupt its form in a way normal spells can't.
One possibility is that it could be an inheritance. Maybe he discovers he has a distant relative who left him a piece of land. Another could be that he is rewarded with land for his heroic deeds. For example, the wizarding community might give him a parcel of land as a token of their gratitude for saving them from Voldemort.
He could lose the trust of his friends. In the original series, his friends rely on him to be a force for good. If he starts using the Dark Arts, they would be horrified and might distance themselves from him. Also, he could be in danger of being corrupted by the dark power, which could change his personality and make him more like the villains he once fought against.
In some fanfictions, Harry might use a flask to store potions. For example, he could carry a flask of Felix Felicis, the luck potion, with him during a dangerous mission at Hogwarts. This would give him an edge in tricky situations.