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.
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.
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.
His outfit might change a bit. He would need some kind of holster or attachment for the lightsaber. Maybe he would add some elements to his clothes that are related to the Star Wars universe, like a Jedi - like robe or a belt with some Star Wars - style buckles. His hair might also look a bit different when he's using the lightsaber, as if the power of the weapon gives him a more intense look.
His fighting style would likely incorporate more acrobatic moves. Since the Keyblade can be used for things like jumps and dashes in mid - air, Naruto would start using these abilities. He might also use the Keyblade to block attacks more effectively than he could with just his kunai or shuriken. And he could use the Keyblade's power to enhance his speed, allowing him to quickly close the distance between him and his opponents or dodge attacks more easily.
Naruto could combine his Shadow Clone Jutsu with Stormbreaker. He could create clones to surround his enemies and then use Stormbreaker to launch a powerful attack from the center. This would be a new and unexpected tactic for his opponents. Also, he might use his chakra to enhance the power of Stormbreaker even further, like making it shoot out energy blasts.
His lightsaber might have a blade that is constantly changing in shape and intensity, just like his unpredictable ninja skills. The hilt could be made from a special metal found in his world, perhaps with a chakra - infused gem at the center. The color of the blade could be blue, representing his loyalty and trustworthiness, and it could also emit a faint glow that corresponds to his own chakra signature.
He'd first have to get used to the weight and balance. Naruto is used to his kunai and shuriken, which are much lighter. The lightsaber's energy blade would also be a new concept for him. He might start by simply holding it and getting a feel for it, like he did when he first learned to use a sword in the original series.
Naruto's fighting style would likely become more focused on close - combat. Spartan armor is designed for hand - to - hand and melee combat. So he might rely less on his long - range ninjutsu and more on using his fists, a Spartan - style shield, and a short sword if the armor comes with those. He would also probably become more defensive, using the armor's protection to his advantage and waiting for the right moment to strike.
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.
Naruto would likely become more versatile. With the blue lightsaber, he'd have a new weapon that requires different combat skills. This would force him to be more strategic in battles. For example, he'd need to learn precise strikes instead of just relying on his brute force and ninjutsu.