Well, in such a fanfiction, Naruto might use his chakra in a unique way to power the lightsaber. Maybe he'd combine his ninja skills with the new weapon. For example, he could use his speed to make quick slashes with the lightsaber, like his fast kunai attacks.
Sasuke could be one to help. Given their history and Sasuke's intelligence, he might contribute some knowledge about energy manipulation which could be useful for the lightsaber's power source.
Well, in this fanfiction, perhaps Percy could use his water - bending abilities to shape the kyber crystal. He might draw on his demigod skills to assemble the various parts of the lightsaber with precision. After all, he's used to handling weapons like Riptide, so adapting to building a lightsaber could be an interesting journey.
Well, perhaps Harry Potter discovers some ancient magic that allows him to combine the wizarding world's technology with that of the Star Wars universe. He might use his knowledge of spells to power the lightsaber in a unique way, like using the Lumos spell to create the blade instead of the traditional kyber crystal. He could then face off against Voldemort with this new weapon, creating a whole new dynamic in their battles.
Maybe an old wizard who has been secretly studying the combination of different magical universes for years. This wizard could have made some initial attempts at building a lightsaber - like weapon in the past and can share his experiences and mistakes with Harry. He would teach Harry about the balance between the light and dark side of this new form of magic, just like the Jedi teachings in Star Wars.
Perhaps it's given to him by a mysterious figure. This figure could be from another dimension or a very old ninja who has been in hiding for a long time. They recognize Naruto's potential and power, and decide that he is the one worthy of wielding the black lightsaber. This could be part of a bigger plan to help Naruto defeat a new and powerful enemy that neither the ninja world nor the normal means of combat can handle.
Naruto's fighting style would likely incorporate more acrobatic flips and spins to take advantage of the lightsaber's reach. He'd also use his shadow clone jutsu in new ways, like having clones surround an enemy while he attacks from above with the lightsaber.
Another possibility is that the black lightsaber has some sort of special power or property that ties into the Naruto universe's existing powers. For instance, it could be attuned to chakra in some way. Maybe it can be powered by chakra instead of the typical energy source in the Star Wars universe. This would create an interesting dynamic as Naruto characters would have to learn how to manage this new form of power while still using their own ninja abilities.
He could find it during one of his missions. Maybe he's exploring an ancient ruin and stumbles upon the lightsaber. It could be in a chest or just lying there, and Naruto, being curious, picks it up.
If Naruto creates a lightsaber, it could be related to his connection with Kurama. Kurama's chakra is extremely powerful. It's possible that this chakra could be harnessed in a way that can mimic the energy source of a lightsaber. Naruto, being the creative and adaptable ninja he is, would then figure out how to shape and control this new form of energy into a blade, much like a lightsaber. And he would surely use it to protect his friends and the Hidden Leaf Village.